This better shows the semantic of the API. For already setted fields,
mergeFromString do replacement for singular fields and appending for
repeated fields.

1. Generate single file for each message.
2. Lazily initiate metadata.

The last up-integration from github to google3 was made at commit
e28286fa but somehow the change in
that commit is not actually in the up-integration CL and as a result
that change was reverted by the down-integration. Restoring them
should make the jenkins tests running again.

Needed to pick up a Java dependency change and re-populate the offline
repo.

[skip ci]

Needed to pick up a Java dependency change and re-populate the offline
repo.

[skip ci]

1. Added ruby22 and jruby tests to jenkins.
2. Added javascript tests to jenkins.
3. Added golang tests to jenkins.
4. Removed ruby19/ruby20 tests from travis. Support for ruby 2.0 has
   ended since 2016/02/24.
   https://www.ruby-lang.org/en/news/2016/02/24/support-plan-of-ruby-2-0-0-and-2-1/

Change-Id: Ie984b06772335352a4be7067ab2485f923875685

This also updates the version number to 3.0.0-beta4

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

Version number for beta languages is updated to v3.0.0-beta-2.
Version number for alpha languages is updated to v3.0.0-alpha-5.

Protoc, C++ runtime and Java runtime are updated to v3.0.0-beta-1, other
languages are updated to v3.0.0-alpha-4.

Change-Id: Ifb4958e4f371b36f249faa466473f851697edb5c

Change-Id: Iae16dcae833959681a5c8e98764e5d1071e6fdcf

- Remove PY25 cruft.

- Selectively apply cleanups from 'python-modernize':

  - New exception syntax.
  - Use 'six' to handle module renames.
  - Use 'six' to handle text / binary stuff.

This PR covers most of the work from #66 which falls inside `python`
(rather than the Python code generation stuff in 'src').

Added __version__ attr to package so that scripts that check local
packages to see if newer versions are available can work.

Almost all Python packages have a version attr, and the vast majority of
them name it "__version__"