Some default mappings can be set to empty strings

This allows the user to not have a mapping. Also, showing a detailed diagnostic
is now possible through a new command.

Fixes #64.
Making sure filename exists before using it
Don't leave whitespace on empty lines

This was caused by complicated interactions with Vim.

Fixes #15.
Minor stlye fix
Updating vimdoc docs
Extra item in FAQ

This one's about conflicting versions of Python detected.
Formatting fixes for readme
Typo fix
Note about python headers to install guide

Fixes issue #73
FAQ entry about the "nosy" completions
Add jedi-based completion.

This first version only uses the Jedi completion engine after a ".", similar to
how the ClangCompleter works. It is also entirely synchronous and blocks for
quite a while the first time it is called.