Issue #372: Track Bitcoin block count and difficulty

Uses new functionality from openclerk/currencies, and starts work on using
the new upcoming Job framework and the migrations framework for defining
additional currencies.
Generate cryptocurrencies help file from Discovered currencies
Moving Bitcoin definition to openclerk/cryptocurrencies

We will place all cryptocurrency definitions into
openclerk/cryptocurrencies, to reduce the complexity of Openclerk core,
and to also illustrate that it is possible to dynamically load new
currencies through Composer packages.
Block Count and Difficulty migrations can now be generated

Issue #372