Added transaction- and output-related functionality

Added some functions that the blockchain will want to communicate with
the blockchain storage.
Also fixed a couple minor typos/omissions.
Added convenience typedef to index outputs

outputs are often referred to by their transaction and the index at
which they belong to that transaction.  This commit adds a typedef to
refer to such a pair <hash,index> consistently.
Blockchain::scan_outputkeys_for_indexes changed to use BlockchainDB
m_blocks.size() --> m_db->height()