Drop Emacs 23 compatibility

* Replace `cl-case' with `pcase' in some places: it's more hip and somewhat faster.
Drop support for `crop'
Use `condition-case-unless-debug'
Add Package-Requires header
Bump version
Initial support for asynchonous backends (#62)
Remove `company-locate-dominating-file'
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
Lower company-idle-delay
Make version string compatible with Emacs < 24.4

`version-to-list` doesn't have snapshot support before 24.4. The `-cvs` suffix
is the closest we can get there and it will be interpreted as a snapshot version
on 24.4+ anyways.