Build changes for ML lib

This addes a pom.xml for building ML lib with Maven. Also I found that ML lib and bagel were not included in the SBT assembly and this change fixes that.

A couple of questions that I ran into:
@matei -- Should examples be included in sbt assembly as well ? 
@velvia -- I also found that make-distribution.sh only builds repl/assembly and so streaming, mllib etc. are not included in the dist jar.  Is this the intended behavior ?