Rename lgamma to log(abs)gamma

[Sumegh-git]

Ping @simonbyrne .
If the changes are valid and right to do, I'll add another commit for doing the same for lbeta and lbinomial, or maybe I can make those changes in a separate PR.
Fixes #154

[simonbyrne]

So I noticed that logabsdet actually returns a tuple of log(abs(det(x))), sign(det(x)). I'm not sure if we want to do the same thing here, or would it be better as a separate function?

[Sumegh-git]

@simonbyrne made the changes!!

[Sumegh-git]

@simonbyrne made the other changes except for removing logabsgamma(x::Number) since that will break lbeta and beta tests. They previously called lgamma_r which worked with Complex{Float} but now we deprecated it to logabsgamma which if I remove (x::Number) will break it.

Also I can't replace lbeta calls with loggamma instead of logabsgamma beacause it also requires the sign(gamma(x)) which is given as a tuple.

So, one possible fix can be to make loggamma return a tuple too. Any suggestions?
Or rather explicitly find the sign of gamma(x) and use that in computation of lbeta instead of tuple?

[Sumegh-git]

@simonbyrne issue is resolved I guess. Renamed lbeta accordingly as well.

[simonbyrne]

Overall, this looks good to me. I'll give it a day or two to see if anyone else has comments.

[Sumegh-git]

@simonbyrne this is fine now! Updated the docs too.

[simonbyrne]

Thanks for your persistence here, looks great!