Add unittests, cleanup setup.py

Add tests that uses unittest, and HTTPretty. 
Switch to setuptools, so we can get tests_requires.
PEP-8 updates
Remove download link, as it was wrong, and it is not needed for pip installs from git. Can be put back in if/when this package gets published to pypi.
Changes package namein setup.py from FrappeClient to frappeclient (PEP-8)