Add GUI (incomplete)
Throw IllegalArgumentException instead of InsufficientFundsException in setCreditLimit(BigDecimal)
Move the source files so as to line up with the package structure in the other branches
Complete package creation and downgrade to language level 6.0
Tentatively merge in the current changes in master, but discard gui's changes to the model for now
Fix the more minor remaining issues
Add back the enum for Account types, and update the GUI to work with it
Merge remote-tracking branch 'origin/master'