Add classpath based lookups to FileUtils

[agur]

• Added more listFile convenience methods.
• There are additional usability improvements possible, just added a few for now.

[agur]

• Created a JIRA ticket IO-473 for this one per these instructions.
  Yet my commit and comment did not yet include a IO-473: prefix.
• Yet not seeing too many pull requests. Thus will also email this to the dev list.
• Also I can't assign IO-473 on my name for fixing. Maybe needs to be a contributor.

[kinow]

Hey @amirgur1 ! Thanks for your contribution. I have edited the issue in JIRA, but since I am not really familiar with [io] I will let others review and comment on your pull request. Thanks a lot!

[britter]

Every public method should have JavaDocs explaining the behavior.

[britter]

Please add some JUnit tests for the new functionality

[krosenvold]

This pr will require unit tests to be merged !

[garydgregory]

-1 needs unit tests. See also discussion items in the Jira ticket.

[agur]

-1 needs unit tests. See also discussion items in the Jira ticket.

Mentioned on Jira, adding tests isn't hard and agree it's needed. But also see Jira discussion and can further cont. there. To ensure we do add this if it's indeed a missing functionality.