Updated Gliese 676.

Added Gliese 676 Ac (RV trend).
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
Merge pull request #383 from ajtribick/issue230

Updated Gliese 676.