(This still doesn't fix the conformance tests, but at least
we're now consistent with the C++ code.)

This also updates the version number to 3.0.0-beta4

We now just perform the optimization within AddRange itself.

This is a breaking change in terms of "drop in the DLL", but is
source compatible, which should be fine.

(Also fix a few more C# 6-isms.)

This fixes issue #1730.

Move to dotnet cli for building, and .NET Core (netstandard1.0) as target platform

This also updates the version number to 3.0.0-beta4

* Improve exception throwing implementation in collections

* Implement RepeatedField.AddRange.

This fixes issue #1730.

* Optimize AddRange for sequences implementing ICollection

(Also fix a few more C# 6-isms.)

* Remove the overload for Add(RepeatedField<T>)

We now just perform the optimization within AddRange itself.

This is a breaking change in terms of "drop in the DLL", but is
source compatible, which should be fine.

This is the only call to TypeExtensions.IsValueType, so we can remove
that method, making the whole type conditionally compiled out for .NET 3.5

* Adding condition compiler symbol to support .NET 3.5

This isn't useful to most users, but can be handy in advanced use cases,
as requested in #1465.

Also added a standalone formatter test, for confidence.
Have validated that undoing the change in 835fb947 breaks the tests
(i.e. we are still testing that the change is required).

(And likewise ignore the prefix in unpack.)

Fixes issue #1459.

JSON tests fail, as we're not using OriginalNameAttribute yet.

(NCrunch noticed these.)

Conflicts:
	csharp/src/Google.Protobuf/JsonFormatter.cs

This fixes issue #679 and issue #1282.
(The .gitignore change is just around ncrunch; I can put it in a separate PR if you really want.)

This was in an attempt to fix the wrapper handling corner case, but it's really fiddly.

This detects:
- An end-group tag with the wrong field number (doesn't match the start-group field)
- An end-group tag with no preceding start-group tag

Fixes issue #688.

The usage of ICustomDiagnosticMessage here is non-essential - ToDiagnosticString
doesn't actually get called by ToString() in this case, due to JsonFormatter code. It was
intended to make it clearer that it *did* have a custom format... but then arguably I should
do the same for Value, Struct, Any etc.

Moving some of the code out of JsonFormatter and into Duration/Timestamp/FieldMask likewise
feels somewhat nice, somewhat nasty... basically there are JSON-specific bits of formatting, but
also domain-specific bits of computation. <sigh>

Thoughts welcome.

"valueField": null

is parsed appropriately, i.e. that it remembers that the field is set.

Previously we were incorrectly packing wrapper types.
This also refactors FieldCodec a bit as well, using more C# 6-ness.

- Spot an Any without a type URL
- In the conformance test runner, catch exceptions due to generally-invalid JSON