global XKeyboard variable my throw

global XKeyboard variable xkb will throw uncatchable exception if X is
not available. This will make vim SIGSEGV: see this issue:
http://github.com/lyokha/vim-xkbswitch/issues/5. In this commit a
wrapper class was introduced that will catch exceptions from XKeyboard()
minor cleanup
Merge pull request #4 from lyokha/master

vim API: global XKeyboard variable my throw