Move jedi_vim.py to ftplugin folder and fix paths

Jedi-Vim is at heart an ftplugin. This is the first step to phase out
the need for a plugin/jedi.vim file.
Move Python 2/3 compat function into autoload

The Python command is only used in autoload/jedi.vim anyways, so there
is no need to set it up globally in plugin/jedi.vim.
Remove the need for a non-filetype plugin

Everything that was set in plugin/jedi.vim can be safely set in
ftplugin/python/jedi.vim instead, thus obviating the need for global
initialization. Additionally, because no global changes are made, the
check for g:loaded_jedi can be omitted as well.

Apart from that, all functionality from plugin/jedi.vim has simply been
copied over into ftplugin/python/jedi.vim.
Remove duplicate omnifunc setting
Put comments in their place

- Removed py_fuzzycomplete.vim header
- Un-derped header locations