Added X11/Hirocoin support

[n00bsys0p]

Added X11 support via Evan Duffield's xcoin-hash Python module, along with a policy for X11 chains.

Also added specific settings for Hirocoin as an example X11 coin. This required changing the DB column chain_code3 to CHAR(4), as Hirocoin has been changed to "HIRO". Not sure if this will require a search and replace of code3 to code4?

[jtobey]

Thanks! I think you need two small changes. Instead of CHAR(4) make it VARCHAR(4). And add

if policy == "Hirocoin":        return Hirocoin(**kwargs)

to Chain.create, please.

[n00bsys0p]

Committed now.