make error message easy to understand

now developer can understand that JDK7 is needed to compile.
rename targetPackages to targetPackage

other parameters don't use plural form, so it is better
to rename it to keep consistency.
close #7: now command line argument also can specify targetLocation.

to support distributing files by CD-ROM etc.,
targetLocation should be specified by command line argument.
fix: git module should use targetLocation.