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.