Abstraction over random number generators

* Introducing module "Main.Random".
* Requires Rank2Types.
Documentation updated in Text.Pwgen module
Enabling to use mwc-random instead of MonadRandom

As a result password generation is faster and mwc-random library states
that it generates values with uniform distribution.

Introducing flag -fwith-mwc-random (default) that forces dependency on
mwc-random package.
Bumping version to 0.4.4.0