Ensure that FieldMask, Timestamp and Duration ToString() calls don't throw

Fix bazel BUILD for Java.

Refactor README.md.

Move the original README.md to src since it's talking about C++
exclusively and add a more general README.md to document how
to install protoc for all languages.

Fix mistakes in highest-representable float64 integers

Avoid upcasting uninitialized pointers

Fixes google/protobuf#693

msan flags this as being undefined behavior. I think it's triggering
because the compiler has to insert a branch to avoid changing the
pointer's value if it starts out NULL. I can't figure out if this is
actually undefined behavior or not, but it definitely seems to be a gray
area of the standard which is best avoided.

Fixing compile errors on Solaris in 64-bit mode

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.

Introduce ICustomDiagnosticMessage to allow for custom string formatting

Fix compiler warning from repeated_field.h

Update podspec version to 3.0.0-beta-2

throw IOException instead of InvalidProtocolBufferException when appropriate

JSON Conformance improvements

Fixed Python 3.x C++ build, and updated conformance failure lists.

It's already submitted.

Fix tests broken by the Java directory change.

"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.

Change-Id: I39c4eadbd28164cfd83aa2d1d8b6dfe8286db803

Restructuring protobuf to multiple modules