add t distribution and robust EP

[yorkerlin]

@karlnapf
It will be great to add t distribution and robust EP in Shogun
Note that the existing implementation of t distribution does not support EP inference.

reference
paper: http://www.jmlr.org/papers/volume12/jylanki11a/jylanki11a.pdf
code: http://research.cs.aalto.fi/pml/software/gpstuff/

[karlnapf]

Yep, this also depends on finally tackling a proper base class for distributions, #1998, #1929

[yorkerlin]

I took a close look at the implementation in gpstuff.
Note that the EP implementation in shogun does not pay much attention to negative covariance.

[yorkerlin]

this issue is related to issue #1832

[yorkerlin]

reference:
https://arxiv.org/abs/1412.4869