Fixed tests, added filters for group-specific getIteration methods, added getProjectActivities method

Hey, I added filtering for limit and offset to the group-specific getIterarion methods.

Furthermore i fixed the tests and cleaned up (a bit) of the mess in the documentation. (in code & README.md since i cant run pcregrep on osx, so you may want to run make doc and check the output)

Btw: at npm theres an outdated version of this package which doesn't include group-specific methods at all