Fix translator comments being discarded

Due to a minor AST format difference we were discarding all translator
comments. This patch fixes it, thus fixes issue 58.
0.4.4
Stop building for nodejs v0.8 on TravisCI
Added `po-headers` option that can be used to override the default headers.

Example: 

    options["po-headers"] = {
      "project-id-version": "MyProject 1.0",
      "language-team": "MyTeam <team@my.org>",
      "language": "en_US"
    }

Also added a test for custom headers.

Problem to solve:
When jsxgettext is used in an automatic workflow for extracting strings (for instance with grunt-jsxgettext) without the `join-existing` option, the generated .pot file will always contain the default headers.

Since there can be a workflow where the freshly generated .pot file is later merged with .po files, we might want to set project, version and team information already in the .pot file to prevent other tools like poEdit from complaining about missing information.

That's where the `po-headers` option can be used (for instance in the grunt-jsxgettext configuration file).
The comments we "extract" are, well, "extracted comments"

Fixes #59.
0.4.5
Added commandline options -project-id-version and -report-bugs-to for setting .po/.pot file headers.
Added the new cli options to the README
Limit overriding of custom headers to --project-id-version and --report-msgid-bugs-to,
test checks for successful override of these 2 and for a proper ignorance for another header keyword.
Merge pull request #60 from mphasize/master

Added `po-headers` option that can be used to override the default headers
Some clean up after #60.
0.4.6
Fix escaping of quotes in nested handlebars blocks
Merge pull request #62 from zaach/fix-quote-escaping

Fix escaping of quotes in nested handlebars blocks
Extract a parse function out of the generate fn

This allows a third-party user of the library
to use the not yet compiled strings for certain things.
Merge pull request #63 from mila-labs/refactor-gen-extract-parse-fn

Extract a parse function out of the generate fn
0.4.7
Support standard way of translator comments

Closes #61.
Merge pull request #64 from zaach/standard-comments

Support standard way of translator comments
0.4.8