Clarified the import in simmetrica.__init__

In Python 2, this means "from simmetrica.simmetrica import Simmetrica"
but in Python 3 it means "from simmetrica import Simmetrica". To make
this consistent in both versions, import the absolute_import and use
the dot notation to clear import the package relative to the init.
Fixed tests to pass in Python 3.

The simmetrica code uses a dictionary and range in a few places. This
causes some small Python 3 compatability issues.

The order of the dictionary isn't guaranteed, but the test expects it
to be. This fix changes that so it works in Python 3 by sorting the
result before comparing.

In Python 3 range() doesn't return a list, but rather returns a range
object. Therefore, to make this work in both versions the test makes
sure its a list before comparing.
Test Python 2.6, 2.7, 3.3 and 3.4 with Travis

If we use tox to run the tests we can verify against multple Python
versions with Travis. This should make it easier to identify issues
with version compatability.