Basic implementation of KrakenConnection class.

Currently untested and unused.
Refactor common stuff into _query(); rename KrakenConnection to Connection and allow connection reuse.

Most of repeating definitions are moved to _query. URI is still declared
twice: once in API class, once in Connection.

Allow reusing previous connection to api.kraken.com by providing a `conn`
argument to query_public() and query_private(). If no `conn` is given,
then a new connection is automatically opened (but not closed).
Update docstrings and User-Agent string.
Bump version to 0.0.2