POC: Allow customizing encoding of known types in the C-extension. Fixes #192

[vamega]

The issue is described in #192.

Some notes:

1. I don't know if this works on a Big-endian machine. One of the commits that was involved in breaking this specified was related to an issue on big-endian systems. I don't have access to a big-endian machine, and haven't invested time in setting up an environment where it can be tested.
2. This doesn't handle the discrepancy between the value that reading the canonical field has. Right now it's always read as True or False.
3. It also doesn't add any documentation of this parameter, which is probably important if it's to not regress.

[coveralls]

coverage: 93.069%. remained the same
when pulling 2bd36b8 on vamega:master
into a195c8e on agronholm:master.