Break dif code into it's own library

[pridkett]

In reality this project is become two different projects. The first is dc2uddf which is the user facing program that downloads dive data and invokes the methods to save it to uddf. The second is dif - the dive intermediate format - which is a utility library for storing dive information in C programs. dif also includes code to serialize dives and perform certain algorithms on dives.

For the sake of clean code the code for dif should be broken out into a small library that can be reused. This will ensure that the interface between dc2uddf and dif stays clean and makes the project more maintainable.