A few fix-ups for protoc-artifacts

Include all well-known type protos.

Change-Id: I122f1cee71a8a739ea603e52582cb0fa9698f0ed

1. make google/protobuf/stubs/pbconfig.h before making protoc, otherwise it
won't build a freshly checked-out code.
2. Document the build environments that have been tested to work.
3. Add support for MINGW64

This is necessary for testing on iOS.

Change-Id: I54ec1e3aa2e9fbfff9a6cd6580920a6a62069b63

Fix for issue 283: JSON handler cleanup typo causing segfault.

Includes repro test case from @wfarr.

Migrate Python tests to stdlib unittest, drop apputils dependency.

Use stdlib's 'unittest' instead.

We already run all tests with and without `--cpp_implementation`

Added Python to Travis build and clarified setup instructions.

Process to deploy artifacts for multiple platforms into a single release.

Remove useless file that doesn't load

This file was broken in ada65567.

Change-Id: Ib75664194491643f8e4f1503a2ed942a2d1e1655

release.

- Do not close the staging repository automatically
- Added staging.repository property
- Updated README with instructions for deployment
- Fix building 32-bit Mac artifact

perf: String#getBytes(Charset) vs getBytes(String)

Better support for cross-compilation

Build scripts to publish precompiled protoc binaries (Maven-based)