preliminary support for MINLP

currently solvers like KNITRO require the provisioning of variable
types (Bin, Int, Cont, etc) up-front, and cannot modify them after
problem initialization, so they cannot support MINLP formulations
through setvartype!

Currently, I’m providing a workaround by dispatching based on the
function signature of loadnonlinearproblem. If it accepts variable
types, then we do not assume that the variables are continuous.
Revert "preliminary support for MINLP"

This reverts commit 3e6444a1d2fa22b01fff8cd465ca340b5f5e8fbf.
preliminary support for MINLP
remove typo
add test for MINLP
list solvers supporting minlp
remove redundant comment
some bike shedding
fix warning for minlp test
support for @addNLConstraints (close #332)
update news and make test a lil more useful
support discrete variables in nonlinear interface