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
Add install.sh for easily building
No need to build the tests
Add friendly clang selection using cmake
Copy libclang inside the python dir
Update install.sh with --clang-completer option

Also gives better messages for install instructions for homebrew.
Respects USE_CLANG_COMPLETER flag in CMake build

If USE_CLANG_COMPLETER is ON, will try to use the system clang, and if
the version is not the minimum required or not present, download.