Docs for Homebrew Python/MacVim

This can't really be fixed on YCM's side since it's a system configuration
issue, but we can at least warn the user and provide him with information on how
to resolve this.

Fixes #18 (not really, but it's the best we can do).
Move the save_cpo lines higher

Somehow they drifted downwards. Fixes #56.
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 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.
Fix usage on install.sh
Update the README mentioning install.sh