Major Description
Originally part of AVRO-152, this introduces getStringOptional and getStringRequired helper methods, to facilitate throwing meaningful exceptions when something's wrong with a schema.
There aren't tests that bad schemas fail the validation in the expected ways, but this did trigger a test which had a fixed field of size 0, which makes no sense; that has been fixed.
Where it was immediately adjacent, I've fixed brace-less if's according to the style guide (http://java.sun.com/docs/codeconv/html/CodeConventions.doc6.html#449).