Further testing

I change Standard.md to be an actual markdown file.

I renamed Transaction.Output and Transaction.Input to SiacoinOutput and SiacoinInput respectively, because there are siafunds as well.

I added a forking test to make sure the state rewinds and reapplies correctly (it didn't, but now it does).

I added contract testing, transaction testing, and expanded the crypto package a little bit to check for nil stuff.

I changed to using encoding.Marshal and encoding.Unmarshal instead of doing it manually. This may have introduced regressions, but only in packages that have minimal to no testing. The bugs should pop up immediately once we add testing.

The consensus testing suite is a bit of a mess right now. The functions could be more oragnized, and it could use better tooling in general. I've decided to let it be messy for now though, because the testing is now pretty thorough (74%!) and we don't need to test it to death before we return to working on the user experience. I think we can come back in a few weeks and do organizational cleanup, and improve the testing tooling.

Pretty much all that remains at this point is the 100 block limit and the siafunds, and testing for each. (We also need to test the exported functions).