Add AssumeValidArg enum and correct verify_script

[JoseSK999]

I have added the AssumeValidArg enum to make the disabled and hardcoded options explicit, as discussed in #126.

Also I have changed verify_script to check whether there's an assume-valid hash, and if so also check if the block is in the best chain (now, if it is forked we won't assume the scripts). We no longer use the assume_valid field height, as we directly fetch the disk header (in order to verify the state of the block).

Updated the two examples. In florestad/src/main.rs I add a temporary conversion to AssumeValidArg, in next changes we could modify slightly the Command enum such that the assume-valid argument has this type.

[Davidson-Souza]

ACK 76ee39e. Pretty cool approach, looks a lot cleaner now.

[JoseSK999]

Thanks!!