Fixes and a couple new covariance functions

- change cov func signatures from (X, Z) to (X, Xs) to follow
notation in lit more closely (where X_star -> Xs, and a gp
from inducing points usually denoted with a Z
- fix to behavior of active_dims
- rewrite of merge_factors fixes bug involving multiplication
of a cov_func by a RV
- adds WhiteNoise and Periodic covariance functions
- some pep8 and style fixes

Author: bwengals