Ponylang support

[locksfree]

Hello Shougo,

Please consider this PR for the Pony language (http://www.ponylang.org/). It does not support generating ctags for now so this simply extracts the actor/class/interface/trait/primitive and their members.

Here is an example of an outline for this file of the pony standard library:

 - _PayloadRequest : primitive
 - _PayloadResponse : primitive
 - _PayloadHeaders : primitive
 - _PayloadContentLength : primitive
 - _PayloadChunkStart : primitive
 - _PayloadChunk : primitive
 - _PayloadChunkEnd : primitive
 - _PayloadBody : primitive
 - _PayloadReady : primitive
 - _PayloadError : primitive
 - _PayloadState : type

 - _PayloadBuilder : class
   + new request(...)
   + new response(...)
   + fun state(...) : _PayloadState
   + fun [ref] parse(...)
   + fun [ref] done(...) : Payload^
   + fun [ref] closed(...)
   - fun [ref] _parse_request(...)
   - fun [ref] _parse_response(...)
   - fun [ref] _parse_headers(...)
   - fun [ref] _parse_content_length(...)
   - fun [ref] _parse_chunk_start(...)
   - fun [ref] _parse_chunk(...)
   - fun [ref] _parse_chunk_end(...)

I didn't see any CONTRIBUTE so please let me know if there is anything that should be changed.

[pvirmaud]

@Shougo modified as requested.

[Shougo]

LGTM