Make perspective mode work along with projectile.

I've added file persp-projectile.el to set up the bridge between perspective mode and projectile. I use both awesome libraries heavily on my daily basis with many projects open at the same time, and found it useful to keep each project related files in a separate perspective for clarity, similar to how I organize windows in different desktop.

In persp-projectile.el, I defined a macro to create before advice for function `projectile-dired` and `projectile-find-file`, such that just before those two functions get invoked, `persp-switch` will be called to create a new perspective and switch to that. Then later, when `projectile-persp-switch-project` gets called, Emacs will switch to the corresponding perspective for that project instead.