Add fit method

[johnmyleswhite]

This moves the fit method to StatsBase. Will need to sync up with changes to Distributions that I'll prepare in a moment.

[simonster]

Thanks for doing this. We should export fit, too.

[simonster]

Actually, maybe we shouldn't export any of the symbols for functions on StatisticalModel/RegressionModel. They're not useful on their own, and any module that defines these methods will probably want to re-export them anyway.

[johnmyleswhite]

I'm really torn about exporting in general because of the absence of a re-exporting mechanism in Base.

[simonster]

I agree that we need a better general solution for re-exporting; I have proposed a couple only to realize that reaching an agreement on syntax is difficult. In StatsBase in particular, there's a lot of only vaguely related functionality that should probably live in separate submodules, but for now it's annoying to split it up because of the state of JuliaLang/julia#1986.

In this case, however, I don't think re-exporting is a huge problem since there aren't too many methods and modules that define types that inherit from StatisticalModel will only want to export the symbols for the methods they define anyway.