Updated Gliese 676.

* Added Gliese 676 Ac (RV trend). Related issue: #230
* Updated discovery dates to publication dates of relevant papers.
* Added error bars.
* Fixed mass of Gliese 676 Ae.

Planet values from: [Anglada-Escudé & Tuomi (2012)](http://adsabs.harvard.edu/abs/2012A%26A...548A..58A)

Discovery information for Gliese 676 Ab, c: [Forveille et al. (2011)](http://adsabs.harvard.edu/abs/2011A&A...526A.141F)