Check for valid API key, dataset as an attribute

[beechnut]

This is an awesome start of a gem, and I'm looking to contribute to make it more human-friendly.

• Client#initialize uses hash to initialize dataset (SF1, ACS5, what have you)
• Client#initialize if it's passed an invalid API key
• Dataset is a malleable attribute of the client, so:
  • the user doesn't have to pass it for every request
  • the user can change it when desired
• Made gem's file loading dependent on the lib/census_api directory, instead of the previous array of filenames

I updated the spec to reflect these changes, but tests are failing because of the pre-initialization RestClient request that VCR doesn't know what to do with. I'm not familiar enough with VCR to debug this at present.

[tyrauber]

Fixed specs. Bumped to version 1.0.2 and pushed to rubygems. Thanks.