Testing foo instanceof Context is fugly

[soci-viii]

I've been using foo instanceof dust.makeBase({}).constructor: that seems clunky. I'd like a way to get the constructor directly, without having to instantiate an object: foo instanceof dust.Context.

Eliminating makeBase() altogether might also belong on the table here? Swapping the stack and global arguments would make Context operate very much like makeBase does now. Further, no external code at present relies on Context() directly; the only change required would be adding a couple null or undefined leading parameters in dust.js proper.

[soci-viii]

I've posted commits for both resolutions. Thanks for your consideration.
https://github.com/socialtables/dustjs/commit/342d99cf3c90c92ca5d4dcc43782b7a900419a3f
https://github.com/socialtables/dustjs/commit/f00b69595f62e4c71627f6bb4e6a879272af4eec

The second has a pull request you are welcome to take if the change pleases you:
#53