A NULL pointer returned by libclang shouldn't be compared against None.

This fix #299.
Index.parse doesn't return None on failure.

It raises a TranslationUnitLoadError.
Fix canFindBuiltinHeaders if index.parse fails.
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
Try to jump to definition before trying to go to the declaration.
Merge pull request #311 from Rip-Rip/goto-definition-same-tu

Try to jump to definition before trying to go to the declaration.
Objective-c++ files were considered as c++ files.

Fix issue #313
Workaround issue #297
Only map <CR> when needed.
Merge pull request #319 from kirneh1986/master

Only map <CR> when needed