updating to spec 1.0

This commit includes a number of significant changes:

  - updating the library to use the new spec
  - moving to analytics.write_key API
  - moving to a consumer in a separate thread
  - adding request retries
  - python 3 support
  - making analytics.flush() synchronous
  - adding full travis tests
Merge pull request #25 from segmentio/spec/update

updating to spec 1.0