provide support for minlp

from KNITRO.jl interface
support MINLP through MPB

by multi-dispatching on the existence of variable types in
loadnonlinearproblem
update example for minlp through JuMP
code refactoring

moved some fields from KnitroProblem to KnitroMathProgModel;

MPB interface: problem gets initialized/loaded only during solve time
type annotations for fields in KnitroMathProgModel
replace Integer with Int
Merge pull request #15 from JuliaOpt/minlp

add support for MINLP through JuMP