Use properly pod2usage to print help

This will add consistency in the help display, becuase the current string is out of sync with the POD format. We should check also carefully the [API of pod2usage](http://search.cpan.org/~jhi/perl-5.8.1/lib/Pod/Usage.pm#pod2usage()).

[x] Remove unused string "$usage" and wherever it appears
[ ] Consistent exit value for errors (see API -exitval)
[ ] If packages thrown an error which requires help display, I recommend to just return false and delegate to distmap the error handling for consisten workflow. If this is not desired , use -input (and maybe -pathlist) from the API to choose distmap as source of pod2usage.
[ ] Consistent message. Either prefixed with ERROR or not. I guess that error should be a keyword for real errors in the program, and the other is just and user-error. Maybe something like: USER ERROR might be better.
[ ] Use -verbose for different levels of command parsing, etc.

As an example of code for doing the command line, see https://gist.github.com/sirhc/621139

robmaz / distmap