refactor lmnn code

[wdevazelhes]

Description

LMNN's code could be refactored to be more simple to read (see comment #201 (comment))

see also #201 (comment)

[zdk123]

Borrowing this issue to request that the LMNN class be refactored to take triplets, as un the SCML code. The current LMNN class could really be named LMNN_Supervised, following the conventions of this package. I could take a stab at this unless there is some reason not to...