Switch to pybitcointools RE for public key (and other) asserts

[mflaxman]

https://github.com/wizardofozzie/pybitcointools/blob/8b608c538d9f98bdebad692c253ab14a81a3c4d5/bitcoin/pyspecials.py#L12-L27

Thanks for the tip @wizardofozzie

[wizardofozzie]

I'm going to take a look at the blockcypher code properly today, so I'll be able to chime in more often.
I've been using my fork of pybitcointools as a kind means of consolidating all the most useful API functions, whereas I should be just working towards one great API, and blockcypher has the best code by far. It makes a huge difference IMO, so congrats to @mflaxman et al