Python 3.x compatibility fixes

[bjornpost]

This fixes a couple of things to make sure the lib runs on Python 3 (tested on 3.4.2):

• Implicit relative imports (import clockwork_http) have been made explicit (from . import clockwork_http)
• Mixed use of indentation in one file (spaces and tabs), everything is converted to 4 spaces.
• Urllib2 does no longer exist in Python 3 and is split in urllib.request and urllib.error. Fixed imports accordingly in a backward compatible way.

[bjornpost]

Some refactoring ideas:

• Load ApiTests.api_key from the current env (i.e. import os; os.environ['CLOCKWORK_API_KEY']), hardcoding it makes it easy to commit your api key by accident.
• Throw an IllegalArgumentException when there is an unrecognized attribute in the attributes object in API.__build_sms_data()

[garethterrace]

Hi Bjorn,

Really appreciate the work you've put in for us here - drop us a mail at hello@clockworksms.com and we can give you some free credit as a bit of a thanks.

We'll be pulling in these changes as soon as we can and the refactoring ideas we'll take a look at.

Thanks again

[wlcx]

Glad I checked the PRQs before I did this myself! 👍

[garethterrace]

Happy with this - we'll update the package as soon as possible.