This commit implements reject handling as defined by BIP0061 and bumps the
maximum supported protocol version to 70002 accordingly.
As a part of supporting this a new error type named RuleError has been
introduced which encapsulates and underlying error which could be one of
the existing TxRuleError or btcchain.RuleError types.
This allows a single high level type assertion to be used to determine if
the block or transaction was rejected due to a rule error or due to an
unexpected error. Meanwhile, an appropriate reject error can be created
from the error by pulling the underlying error out and using it.
Also, a check for minimum protocol version of 209 has been added.
Closes#133.
