Switch gdp to use pocket-lint

[Curtis Hovey]

GDPFormat's rules were extracted to a standalone library names pocket-lint

The imports
    from gdp.formatdoctest import DoctestReviewer
    from gdp.formatcheck import Reporter, UniversalChecker
become
    from pocketlint.formatdoctest import DoctestReviewer
    from pocketlint.formatcheck import Reporter, UniversalChecker

Note that the plugin is passing a gedit language id, but the methods require pocketlint.formatcheck.Language (a crude gedit Language implementation). Calling the stating get_language(file_path) method may work.