Cleanup completions/guide code

[poke1024]

The relevant commit here is:
efd9b7b

This is a proposal to cleanup the duplicate completions code.

This would help in integrating #37 in a proper way.

Contains three changes:

(1) DAWG now gets a param completions which lets any base DAWG create a guide
(2) Completions are delegated to a new PyCompletions class; this helps avoid code duplication (e.g. in IntCompletionDAWG). Also, PyCompletions will create the underlying Completions instance in its constructor, which will reduce memory allocations (in master, this underlying class is recreated on each function call requesting a completion). At the same time, you can create multiple instances for multiple threads.
(3) loading and storing are adjusted; as a nice aside, you can load a DAWG and it will automatically load completions if they were stored

Here's an example with the changed API that now involved a completer() method:

import dawg
keys = ["weather", "this", "there", "thus", "another"]
d = dawg.CompletionDAWG(keys)
print(d.completer().keys("th"))

I'd prefer to change dawg.CompletionDAWG to dawg.CompletionsDAWG (added s) and d.completions() for consistency (see poke1024@a50edcd).

[poke1024]

Continuing to work on this on a fork.