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
Minor tweaks to new intall process
mktemp on ubuntu demands X's
Typo fix