Specify new way of configuring DoubleEntry

Also migrate spec from instance variables to using `let`
define is a better verb
Account::Set#define : add a new account
`#define` method for Transfer::Set
Update test configuration
update locking spec configuration
DoubleEntry configuration
Update config example in Readme file
configure the jack_hammer script
Merge pull request #25 from envato/de_config

More succinct configuration of DoubleEntry