supportedcones method

For the conic interface, adds a method to query which cones are supported by a solver. This takes in an ``AbstractMathProgSolver`` object. Example usage:

```
julia> MathProgBase.supportedcones(CplexSolver())
5-element Array{Symbol,1}:
 :Free  
 :Zero  
 :NonNeg
 :NonPos
 :SOC
```

It's not 100% clear if this should be a method for ``AbstractMathProgSolver`` or ``AbstractMathProgModel`` (or both).

Ref JuliaOpt/JuMP.jl#83

@karanveerm @madeleineudell @IainNZ @joehuchette