Updated transaction costs, removed deprecated OP_RETURN reference

- More accurate values to calculate costs of Mastercoin transactions are
used.
- The reference to OP_RETURN in relation to transaction encoding was
removed, because the current form of OP_RETURN is not suitable to encode
all Mastercoin transactions. There old paragrah also includes a
reference to v0.9 which seems outdated, given the current release of
v0.9.2.