Make jedi-vim a "true" ftplugin

I like to set ftplugin-specific options in an `ftplugin/<filetype>/<plugin>.vim` file. For jedi-vim, this isn't possible: Due to being initialized at the "normal" plugin stage, my configuration settings in my `ftplugin/python/jedi.vim` file simply aren't picked up.

This branch removes the "global" initialization stage of the plugin and
a) allows for ftplugin customization at the ftplugin level,
b) makes Jedi a "true" ftplugin and thus hopefully simplifies the codebase a tiny bit,
c) removes the necessity for the autocmd that always resets `omnifunc=jedi#complete` on entering a Python buffer.