Added a customizable variable: projectile-switch-project-action

Depending on the value of this variable the following behavior can be
configured:

If the variable is:

- projectile-find-file: projectile-find-file is invoked in the new
project

- dired: dired is invoked on the projects root-directory.

The default is projectile-find-file.
Merge pull request #154 from bastianbeischer/projectile-switch-project-action

Added a customizable variable: projectile-switch-project-action