Add ability to load a query from file.

HIve queries can now be loaded from a file in the resources folder. This closes #94. Also closes #20.
minor query: can we get some guidelines on `Source`?  It's explicitly described as "to avoid" in the style guide:
https://github.com/CommBank/styleguide

I hear Guava's `Files.toString` is a good replacement, but I'm not 100% clear on the tradeoffs of each:
http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/io/Files.html#toString(java.io.File, java.nio.charset.Charset)