Add a feature to toggle transparency

This sets the initial `alpha` frame parameter so that transparency could
be toggled later on. The level of opacity can be configured from a
user's .spacemacs through the variables `spacemacs-active-transparency`
and `spacemacs-inactive-transparency`.

Here's how it looks like when a transparent emacs is in front of the spacemacs powerline doc:

![screen shot 2014-12-28 at 02 20 17](https://cloud.githubusercontent.com/assets/53690/5561969/8f0aa398-8e38-11e4-8004-0ffb533b011f.jpg)

`t r` is already taken but `relative-linum`, `t t` is free :-)