Add support for upickle

[rleibman]

Hi Matthew! Hey... I almost got upickle working, but I'm afraid I don't understand string interpolation well enough, somewhere I think it's missing a flatten, some of the tests are not passing. Do you mind taking a look at it, please?

Specifically:

[info] - can inject Option values with implicit Writes *** FAILED ***
[info]   [[1]] did not equal [1] (Test.scala:18)

Thanks!

[maffoo]

Thanks for this, @rleibman. I've made some changes on your branch to get upickle support working, please see #16, and in particular the last commit ae7a6b4.

[maffoo]

Merged as part of #16.