Add the ability to invalidate any project's cache

`projectile-invalidate-cache` now accepts a prefix argument. When it's
present you'll be prompted for the project to invalidate.

This fixes #102.
Make projectile-project-name work without a project

This fixes #103.
Update changelog
Fix caching issue (fixes #106)
Fix tracking of known projects (fixes #107)
`projectile-switch-project` now runs `projectile-find-file` instead of `dired`
Remove obsolete test
Drop projectile-root-hook

It's a bad idea to hook to projectile-projectile-root, since it could
have disastrous performance implications.
Drop an obsolete test