Add Helm source for projectile-current-project-dirs

Helm source for listing project directories similar to
projectile-find-dir, but with more actions. Default is Dired.
Update helm-projectile-sources-list priority

Prioritize helm-source-projectile-buffers-list and
helm-source-projectile-recentf-list higher, since those two have smaller
candidate space. Otherwise, those two will never be used because
helm-source-projectile-files-list is a superset of both, and candidates
in helm-source-projectile-files-list are always displayed first, making
the two smaller sources almost never be used, unless explicitly move to
that source by arrow keys which is less convenient.
Allow helm-projectile to be used outside of projects

Currently, helm-projectile contains a separate command for displaying
list of known projects, that is helm-projectile-switch-projects. But,
the command still can only work inside a project.

This change allows helm-projectile to be invoked outside of a
project. When in such scenario, helm-projectile simply displays a list
of projects for jumping. After this change, helm-projectile also does
not need to rely on projectile-switch-project and provides a better
project listing, since it can perform more than one action.
Merge pull request #440 from tuhdo/master

Add Helm source for projectile-current-project-dirs, change helm-projectile-sources-list priority and allow helm-projectile to use outside of a project