Adding dynamic block value support in PPS

This will add the block reward typw to PPS systems. Prior to this, each
PPS value was hard coded to 50. Not only are other coins not based on
this but changing it would require code. Changed it that

* Default for fixed type is config reward
* Default for block type is **previous** blocks amount
 * Fallback for first round is the fixed reward value

See in-line documentation of dist configuration. This should help new
pool owners to get setup and clarify things.

Fixes #308