Invalidate project cache if .projectile is modified

This probably should have tests.
Test projectile-maybe-invalidate-cache + changelog
Correct a bunch of indentation problems
Add projectile-cleanup-known-projects

Go through the list of known projects and remove those which
directory doesn't exist anymore.
Fix typo
Merge pull request #295 from Silex/master

Add projectile-cleanup-known-projects
Merge pull request #290 from dgtized/cache-invalidation-on-dirconfig

Invalidate project cache if .projectile is modified
Check if the file is ignored before adding to cache

* projectile.el (projectile-cache-current-file): Add
  to `projectile-ignored-file-p' as an additional predicate
  against adding the file to the cache.

This fixes a problem of "file.~HEAD~2~" (result of
`vc-revision-other-window') being added to the cache although
".projectile" has:

    -/*~
Merge pull request #296 from abo-abo/master

Check if the file is ignored before adding to cache
Modifies `projectile-ack` to accept a prefix argument
- With a prefix argument, prompts for a directory to perform the ack search
Makes `projectile-ack` ignore `projectile-ignored-files`
Merge pull request #294 from NicolasPetton/master

`projectile-ack` improvements