Reorganize. Simplify. DRY.

Reorganizes basic/common/mvc sub-generators into a simple, single generator.  It seemed to me that the "sub-generator" pattern was not a great fit for what we were trying to accomplish.  MVC is not as much a sub-generator as it is an option on the main express generator. I modeled this flow off of some of the yeoman maintained generators. For this reason I moved it to an option.  For this reason, using the MVC option is now created via `yo express --mvc`.

I'm thinking that a sub-generators are better suited for things like adding new models in the MVC version or perhaps installing SASS/LESS/Stylus scaffolding.

DRYed a lot of the redundant stuff from the previous sub-generators.  Despite the size of the pull request, this was relatively straight forward.  It is mostly just shifting around of files, deleting unneeded files and removing a lot of cruft to simply things now that I have a better grasp on some yo generator internals.  As such, instead of trying to review the diff version of the pull request, I'd recommend peaking at the [full repo](https://github.com/nategood/generator-express/tree/dry).  It's certainly simpler now.

Tests have been updated and are all passing.