Heuristic callbacks
Docs and example for heuristic
Clarification over solver capabilities
Make constraints parametric
Update docs to reflect wiping behaviour
Merge master into heurcb
Merge pull request #110 from JuliaOpt/paraconstraint

Make constraints parametric
Merge branch 'master' into nlp
fix moving expression printing target
clean SOS commit
Merge pull request #112 from JuliaOpt/sos2

clean SOS commit
Merge branch 'nlp' of github.com:JuliaOpt/JuMP.jl into nlp
Merge branch 'master' into nlp
use graph coloring for efficiently evaluating sparse hessian