Fix instance parameter

I used this tool to create import for gi.repository, enabling my PyCharm completion for GTK3.
I noticed the generated stubs was duplicating the *self* parameter with the named one in the .git file, for example, this was generated for the builder class:

    def add_from_file(self, builder, filename):
    ...

instead of:

    def add_from_file(self, filename):
    ...

This pull request adds support for the `<instance-parameter>` tag present in the `.gir` files, but feel free to perform more tests on your binary packages as well, looks odd to me no one ever reported this :)

Thanks for your work!