Updated API, refactored netcode, and added ticker support.

In common.py:
  * Factored out http-request code from `common.py' into `bterconnection.py'.
    ** Preserved backwards compatibility by importing moved symbols
       directly into the `common' namespace.
  * `all_pairs' list is now created at runtime via a request to data.bter.com.

In bterconnection.py:
  * Moved `domain' variable into `BTERConnection' class; it is only
    used by this class.
    ** Changed it from `bter.com' to `data.bter.com' to reflect changes to api.

In ticker.py
  * Added `Ticker' class to request ticker data from data.bter.com.