allow name on target and proto to differ

[jamestalmage]

I propose changing the signature of all methods to take two arguments nameOnProto, and nameOnTarget. The second argument would be optional and default to the value of the first (making it fully backward compatible).

Not sure which order the arguments should be in.

@tj, If you like the idea I'll submit a PR.

[jamestalmage]

Best use case is for name collisions:

delegate(Person.prototype, 'mom').getter('momsName','name');
delegate(Person.prototype, 'dad').getter('dadsName','name');

[jamestalmage]

@tj @dominicbarnes
let me know what you are thinking on #8 as well. If you intend to merge it, then I'll base my next PR off that code (there will be lots of merge conflicts if I start this PR from master and you intend to merge both)