Open and navigate between buffers

This is a huge installment in the series -- one in which I start to
really make this mimic my existing Vim setup.

(Yeah yeah, I know some of you are saying, "Why are you just making
Emacs behave like Vim? That's missing the point." Because I know for
my switch to Emacs to have any permanence, I need to bootstrap my brain
into it, i.e. I need to be productive on Monday. Change is ok;
starting over as a n00b is not. Don't worry, I'll learn more Emacs-y
ways of doing stuff with time. Besides... the main reason I want to try
Emacs again is because I have my own special notions of how I want my
editor to behave and want to customize.)

In this commit, I learned "just enough" about Helm and Projectile so I
can quickly open files like I did with my beloved CtrlP vim plugin. I
know I'm missing A LOT of what Helm can do, but I'll just have to come
back to it later.

My goal here was to get my four main buffer navigation keys working:

,r  open a recent file
,t  open a file in the current project
,y  switch to a different buffer
,/  switch to the last open file (similar to Vim's "alternate-file")

The first three are now very close, and the last one is much
better than I had hoped.

Whereas Vim's alternate-file would only toggle between the last
two files, iflipb (which I have mapped to <leader>/) works more like
alt-tab on my Mac, and it rocks. So yay.

You'll notice I installed evil-leader so I can create some familiar
<leader> key bindings.

There is still much more to do/learn here, but I'm happy with the
progress and can see myself actually maybe using this editor on Monday
when I get back to doing "real" work. Though, I need just a few more
things... stay tuned!
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