[pycmbs/constants.py] module for storing constant values

pycmbs.EarthRadiius becomes pycmbs.constants.EarthRadius

[constants] dedicated file for storing constants

[init.py] moved constants to a separate file, brief clean up (removed
commented lines)

[tests/constants] test file for constants.py

[tests/pycmbs] removed EarthRadius test, moved it to test_constants.py
Merge branch 'master' of git.assembla.com:pycmbs

removed comments, moved constants and merged in mapping modules import

Conflicts:
	pycmbs/__init__.py
[REAMDE] added coverage badge
Merge branch 'master' of https://github.com/pygeo/pycmbs