Add options to control "jump to declaration" key mappings.

g:clang_jumpto_declaration_key sets the key used to jump to declaration.
g:clang_jumpto_back_key sets the key used to jump back.
The default values are <C-]> and <C-T> respectively.
Improve cc_args by supporting warning and std options

* cc_args now save -W* and -std=* arguments
* Minor code cleaning
Merge branch 'master' of git://github.com/daedric/clang_complete into daedric-master