Expose `write_message` function to Lua decoders to support modifying
the existing message w/o having to generate a new one.
Incorporate review feedback, add tests and docs.
Merge branch 'dev' into sbx-write-message

Conflicts:
	CHANGES.txt
	sandbox/lua/lua_sandbox_test.go
Merge branch 'dev' into sbx-write-message

Conflicts:
	docs/source/sandbox/lua.rst
	sandbox/lua/lua_sandbox.go.in
	sandbox/lua/lua_sandbox_interface.c
Add unit tests for write_message
Merge pull request #592 from mozilla-services/write_message_tests

Add unit tests for write_message
Merge branch 'sbx-write-message' of github.com:mozilla-services/heka into sbx-write-message

Conflicts:
	sandbox/lua/lua_sandbox_test.go
	sandbox/lua/testsupport/errors.lua
More review feedback incorporated.
Support fields keyed on empty string, add positive tests for the rest of
the message fields.
Merge pull request #589 from mozilla-services/sbx-write-message

Sandbox `write_message`