0.1.7: new helper module, full avro 1.4-1.9 support (#22)

* new helper module, full avro 1.4-1.9 support

this commit includes the following:
- new helper module build has a sub-module per major version of avro which
  compiles natively vs the target avro version. this eliminates the need
  for reflection in _most_ use cases. cases involving optional avro-compiler
  jar still rely on reflection
- helper code now implemented across all major avro versions from 1.4 to
  1.9 inclusive
- code generation support has been extended to cover above avro versions
- tests are now performed with multiple classpaths, including variants
  with and without the avro-compiler jar (on avro 1.5+)

Signed-off-by: Radai Rosenblatt <rrosenbl@rrosenbl-ld1.linkedin.biz>