Shrinkdesc 20 - Allow import from empty streams for Descriptor Import

[andygibson]

Here's a fix for importing from empty streams. It does it by reading the stream to a string, and then reading from the string after checking to see if it is empty or not. If it is empty, it returns a new empty instance of the descriptor. If not, the descriptor is loaded as normal.

[ALRubinger]

Thanks for contributing this, Andy.

I've taken your work and refactored it a bit to match the current design: https://github.com/shrinkwrap/descriptors/commit/e4ba790c2f530b2abe75ffbd7ca6b33631c348e4