Refactored monos, added many features to posys

 - Created a template 'nomials' class for shared features
 - added 'eid' attribute to monomials (it's basically a hash of 'exp')
 - added algebraic simplification to both monomials and posynomials
 - added posynomial-posynomial multiplication
Yeah, definitely more elegant.  I'll switch posynomials too.