Rename Preconditions to ProtoPreconditions

(Generated code changes in next commit.)

Rename GeneratedCodeInfo to GeneratedClrTypeInfo

Recently, descriptor.proto gained a GeneratedCodeInfo message, which means the generated code conflicts with our type.
Unfortunately this affects codegen as well, although this is a part of the public API which is very unlikely to affect hand-written code.

Generated code changes in next commit.

Clarify readme for C#

Make protobuf Skylark extension appends the workspace root

Manually down-integrate python JSON struct support from internal code base.

Integrate google internal changes to master

Java files are moved to un-do the hack in the prevous commit, which
moved the java files to the original position for integration.

Moving the files to their original location, so that opensource changes
can be picked during the internal merge. Those files will be moved into
the correct location after merging with internal code.

Note: do NOT merge this into master without the other internal
down-integration commit.

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.