Update all references from telldus-py to tellcore-py

I have renamed telldus-py to tellcore-py (for various reason, one being that I didn't want to use their company name directly). This updates all references in tellprox to use the new tellcore-py.

I have also uploaded tellcore-py to pypi: https://pypi.python.org/pypi/tellcore-py

Also removed the usage of sudo when running tellprox from the documentation as
this is not needed. Also added an alternative way of running tellprox without
installing tellcore-py first.