Add support for propositional logic

[hdavid16]

Waiting for Symbolics.jl to support CNF: https://github.com/JuliaSymbolics/Symbolics.jl/issues/59

[hdavid16]

Need to:

• [ ] Create a @LogicalConstraint macro that will accept a logical proposition, convert it to CNF, reformulate it as an algebraic expression, and add the constraint to the JuMP model.
• [ ] Support for AND, OR, IMPLICATION operators