avro

AVRO-2090 second try (#350)

* Finished initial implementation (not tested).

* Added Reader/Decoder code

* Made some of the changes suggested by Doug.

* Hide helper methods related to custom coding.  Changed them from public to protected.  Also changed name of encode and decode to customEncode and customDecode to be more clear as to their function.

* Allow dynamic changes to flag that controls whether or not the custom en/decoders are used.

* Fixed typos in TestSpecificCompiler.java

* New test case: breaks new code-gen when schema needs resolution.

* Fixed bug with decoding when the schema has been migrated.

* Added test-with-custom-coders execution of testing and fixed some problems that this uncovered.

* Fixed potential performance problem of redundantly allocating objects.

* Added documentation (also update AVRO-2090 description to point to new docs).

* Small doc fix (I forgot to commit these changes before pushing)