Support syntax highlighting for multiple languages

Add an optional `language` argument to `pygmentize`, doing a dumb guess as to a file's language. Default language remains objc.
@kattrali on the side note, i think `Snippet` shouldn't be a `String`. This was a quick hack I've done before, but if it was a better object, it'd remember the `file_path` when you initialize it.

The usage:
``` ruby
snippet = Snippet.from_filepath(f[:file_path])
snippet.contents #=> String
snippet.file_path #=> String  /Users/..../file.m:234