Make company-clang work asynchronously

#62
Implement async operation for grouped backends

Closes #62

This probably doesn't have enough error handling, but I can't know for sure
until I see what the typical errors are.
Merge branch 'clang-async'
Update NEWS
company--merge-async: tweak
company--force-sync: change calling convention
Move company-elisp require
Change the summary
Define and use `company-grab-symbol-cons'

Closes #88
company--multi-backend-adapter-candidates: bind backend locally

Fixes #89
Introduce `company-tooltip-minimum-width'

Closes #93