Introduce complete file lists

Hi,

I believe that `gem2rpm` deserves some great improvements for 2015. That means for gem2rpm to be able to generate _by default_ a buildable spec file. The prerequisite for that is to improve the listing of the files in spec files. As I already discussed with Vit, I don't like the idea of having all the logic in templates and then maintain all the possible templates like this. So what I am proposing is to have default methods for file listing that can be part of the core, can be tested etc. and default configuration (that should work for most) that can be overriden in the template (currently as singleton, ideas are welcome).

This would essentialy:
* List all the files from .gem making gem2rpm a lot more useful
* Avoid duplication in templates because the main logic stays in core
* Allow the template to override any default value or simply set its own values at the beginning (here simply as changing the Ruby hash)

It is also important to note that this does not break any backwards compatibily on its own as the templates do not use the file lists methods.

I would like to get your opinion on this and discuss the current implementation that I am proposing. Note that tests are not yet part of this PR.