adding simple validation checks to reggie client

[vsoch]

This will add basic checks to validate the client config, along with brief notes for development in the README to close #24. We discussed checking for an empty UserAgent, but it looks like this isn't possible (at least when called in the NewClient function). I tried various tricks to instantiate the client for the test and then change the UserAgent to an empty string, but always got memory / reference errors. So if it's not possible, we might just remove it. If it's possible, I'd want to know how and then adjust the test.

Signed-off-by: vsoch vsochat@stanford.edu

[vsoch]

We could probably just validate the Address with just the second check (the urlvalidator and skip the check for an empty string) but giving a message about the empty string vs. just an invalid URL seems more meaningful to the user.

[jdolitsky]

awesome, thank you @vsoch !