Kepler-91: confirmed, updated parameters.

Barclay et al. (arXiv 2014): http://arxiv.org/abs/1408.3149
Merge pull request #311 from mistertribs/kepler91

Kepler-91: confirmed, updated parameters.
added planet attributes

added spectraltype and mag(s) to planet
Merge pull request #312 from Sol-d/patch-4

added planet attributes
Added Gl 15. Source: http://arxiv.org/abs/1408.5645 Closes #318