All writers now inherit from a base writer

* the writers are now responsible for cleaning docs
* added an append_json writer that does not delete docs and simply
    modifies index.json
Merge branch 'append_new_docs' of github.com:engineyard/rspec_api_documentation into engineyard-append_new_docs

Conflicts:
	lib/rspec_api_documentation/writers/general_markup_writer.rb
Mention other formats in README