Optimizer for fit method

[aprotopopov]

I found, that optimizer for _fit was changed from minimize to fmin in utils with that commit. I have some questions:

• why Nelder-Mead algorithm was finally chosen?
• why minimize was changed to fmin while there are some notes about that in scipy docs:

  The specific optimization method interfaces below in this subsection are not recommended for use in new scripts; all of these methods are accessible via a newer, more consistent interface provided by the functions above

I'm also interesting in increasing training speed for model (BetaGeoFitter) because in my case I need to wait about 45 minutes to train (~15 mln records). On sample I saw that SLSQP was faster. Could it be used for training model or the are any pitfalls with that?

[CamDavidsonPilon]

I remember we were having trouble getting convergence with very small datasets, hence the change. Your recent changes fix this problem for all parties 👍