Issue16 arbitrary attributes

I have implemented some code to output the arbitrary annotations that are supported in avro idl. These get translated to new key/value pairs of properties on the avro schema. In order to detect them, all we need to do is look for any keys on the object that both avrodoc as well as avro do not generate for the various types of objects.

I also updated the example output to include annotations of the various types that are supported with this patch.

Resolves issue #16

NOTE: There is a commit (851cf994d3138a0544fc9dbcbf029787885f6e42) that I didn't mean to include in this pull request -- this is the commit for the work in issue #14. I reverted that change in (13abdb2a333179d07598d683912ab439a33c9294).  

Let me know if you'd like me to clean this up a bit.

Thanks!

Micah