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

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

protobuf/java will become a parent pom that will contain two modules:

core - contains all of the code for the protobuf-java artifact
util - contains all of the code for the protobuf-java-util artifact

Also cleaned up various Maven warnings.

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

This fixes issue #933, effectively.

Prohibit null values in maps

Allow conformance test runner to tolerate crashes, and re-enable conformance tests