Issue #460 Add sandbox message field iterator
Merge pull request #588 from mozilla-services/sandbox_iterator

Issue #460 Add sandbox message field iterator
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
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.