Resolve #460: Add wallet_scan_progress entry in wallet_get_info output

The wallet now reports its progress as a float in [0,1] any time it
rescans the blockchain. This float is published via the wallet_get_info
output. In the case of some error during rescanning, the value will be
set to -1.
Merge branch 'master' of github.com:BitShares/bitshares_toolkit