new syntax for groups of constraints with addConstraint
Merge pull request #160 from JuliaOpt/addConstraints

new addConstraint syntax for specifying groups of constraints
add missing zero methods
fixup for zero(QuadExpr) also
pre-compile callbacks for better timings
fix typo in docs, thanks to Joel
Merge pull request #1 from JuliaOpt/master

update to b0ef223