Restructure package to improve maintainability and extensibility

[lucianopaz]

compress_pickle started out small, as a single function script that only provided the dump and load functions. I've expanded the functionality a bit and split the package up into different modules but underlying paradigm has always been functional. I feel that the package is now reaching a point where extending it has become hard, without introducing multiple if statement branches and spaghetti code. In particular, #16 will be a bit ugly to get through with the current structure.

For the moment, I don't have the time to restructure the code into a object oriented implementation, which would be easier to maintain and also to extend, so it's likely that I'll first spin out a small patch to close #16, but it would be really nice in the long run, to aim for an OO structure of compress_pickle.

[lucianopaz]

Closed by #26