Refine package validation behavior for prompts

[bfitzpat]

During the work done for #22 we encountered a few additional questions.

• Possibly need to rethink the validation message which is currently. "Unsupported package name. Package must follow standard Java package naming guidelines". @apupier believes this may still potentially lead user to consider current limitations as bugs and not limitations.
• Possibly support package names with no "."? Possibly update the regex to make the "." optional?
• Currently if the user enters invalid input, the prompt returns to a cleared line with nothing remembered from the invalid input. This is the way it works for validated prompts in yeoman right now, but I have opened an issue upstream to see if there's a way to make it work any better (When validating prompt input, invalid text disappears and user must re-type yeoman/generator#1095)

These are all pretty minor issues, so we will see where we go from here.

[bfitzpat]

The Yeoman folks have an issue open for the "kinder, gentler" validation issue here - SBoudrias/Inquirer.js#711 - hopefully it will show up in a later version of the generator code upstream. They closed my yeoman/generator#1095 in favor of the existing issue.