Support Julia 1.0 (and drop Julia v0.6)

Merge pull request #46 from focusenergy/julia-1-update

Julia 1.0 compatibility updates.

Faster simplification of expressions

Thanks to @vtjeng

New function to handle constant objective terms

Merge pull request #41 from rdeits/constant

new function to handle constant objective terms

Better performance

Eliminates internal type-instabilities to improve performance of constructing implications and other constraints.

Fewer redundant variables and constraints

Merge pull request #34 from rdeits/fix-extra-vars

fix warmstart!() introducing new variables

Initial Release

Merge pull request #20 from rdeits/loosen-reqs

loosen version requirements