Expose `write_message` function to Lua decoders to support modifying
the existing message w/o having to generate a new one.
Pull in the latest lua_sandbox 'nan' cbuf changes

- create common cbuf filter aggregators
- remove the reduntant sandbox tests/files from Heka
Add Fields[ResponseTime] to HttpInput

This allows for quality measurements from HttpInput messages. The
HttpInput plugin is now also compatible with the Circular Buffer Graph
Annotation (Alerts) plugin.
(http://hekad.readthedocs.org/en/latest/sandbox/graph_annotation.html)
Issue #460 Add sandbox message field iterator
Merge pull request #585 from mozilla-services/cbuf_nan_support

Pull in the latest lua_sandbox 'nan' cbuf changes
Incorporate review feedback, add tests and docs.
Merge branch 'dev' into sbx-write-message

Conflicts:
	CHANGES.txt
	sandbox/lua/lua_sandbox_test.go
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
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`
Merge pull request #587 from christianvozar/features/http-input-responsetime

Add Fields[ResponseTime] to HttpInput
Don't override RegexpParser default delimiter unless an alternate is
explicitly specified.
Test default RegexpParser delimiter behavior w/ LogfileInput.
Merge pull request #595 from mozilla-services/fix-default-regex-delimiter

Fix default regex delimiter