Implement ppx syntax extension.

Initial patch by Drup.
Implement exception match cases in ppx extension.
Documentation for ppx syntax extension.

and small grammar/spelling fixes for pa documentation.
Fix documentation and wiki generation and opam file.
Ppx documentation improvements.
Use a custom flag to pick native or byte ppx executable.
Decouple try and finally.
Register attributes on `[%lwt ...]` nodes.
Merge pull request #75 from ocsigen/ppx

Ppx syntax extension