Exception handling

Adds exception handling to pymill.

Introduces a `PymillException` as a base class of all exceptions that catches all errors. 

If you want finer control you can catch the `APIException` or a subclass of it.

e.g:

```
try:
    sub =  p.new_subscription(**params)
except SubscriptionAlreadyConnected, e:
    # handle
    print "already subscribed"
except PaymillException, e:
    print "can be anything"
```