Finish documentation for v0.1

[mikhailramalho]

I'd say 90% of the classes and methods are already documented, but I also want to include:

• A couple of small examples (maybe even reuse the regression tests)
• Some documentation explaining the designing decisions, e.g, why we have different sort classes but only one expr class, the *Impl classes.
• A block diagram will be nice (similar to the one in https://github.com/pysmt/pysmt)