Revision 1717850

AVRO-1747: JavaScript: Add implementation.

A few features:

+ Fast! Typically twice as fast as JSON with much smaller encodings.
+ Full Avro support, including recursive schemas, sort order, and
  evolution.
+ Serialization of arbitrary JavaScript objects via logical types.
+ Unopinionated 64-bit integer compatibility.
+ No dependencies, it even runs in the browser.

The previous API is included with deprecation warnings (this adds
`underscore` as a dependency).