Drake function

This implements the "DrakeFunction" idea we'd discussed in #269. I'd like to get feedback about any and all aspects of it, so please, let me know what you think!

This could (probably) be merged in as is without breaking things - everything is contained in the `drakeFunction` package. The only exception is that I had to add the drake root folder to the path in `addpath_drake` in order to get `drakeFunction` on the path.

I might vote for putting it in solvers without the package
On Aug 23, 2014, at 8:50 AM, Andres Valenzuela <notifications@github.com> wrote:

> In +drakeFunction/Constraint.m:
> 
> > @@ -0,0 +1,41 @@
> > +classdef Constraint < Constraint
> I agree - I made that change at the last minute. I can change it back to
> 
> classdef DrakeFunctionConstraint < Constraint
> Then to use it we'd say
> 
> constraint = drakeFunction.DrakeFunctionConstraint(...)
> Alternatively, we could move the class back into solvers as DrakeFunctionConstraint.
> 
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