Better status api for assemblies

So yeah, with this PR we now have more status methods in Assembly (as discussed in #12).

This implements:

* aborted?
* canceled?
* error?
* uploading?
* executing?
* finished?

Please have a look and see if I've missed something. It's missing some documentation adaptions and everything. Also the tests a kinda non-sense.

Well, fire your comments away!

/cc @jeremyhaile