Make projectile-mode-line completely customizable

Don't explicitly set `projectile-mode-line`.  Instead, turn `projectile-mode-line` into a customizable [Mode Line Format](http://www.gnu.org/software/emacs/manual/html_node/elisp/Mode-Line-Format.html), and use an `:eval` construct to show the project name.

This allows users to completely override the appearance of Projectile in the mode line.  For instance, I use the following to show just the project name, but with a catchy face:

```cl
(setq projectile-mode-line '(" " (:propertize
                                  (:eval (projectile-project-name))
                                  face font-lock-constant-face)))
```