code: prevent deoptimization

V8 will create a hidden class when optimizing code, and if any
properties are access that are not on this hidden class, it will trigger
a deoptimization. Simply creating the field and initializing it to null
is enough to address this. JavaScript works better when you pretend
it's statically typed.
errors: fix inconsistencies

There were a bunch of problems with the error tracer:

1. Bananas defaults.
2. Stack traces were used as messages instead of being passed
   as a named parameter.
3. The scope was a non-named path, and was often missing even
   when there was a transaction that was associated with a web
   request.
4. The tracer didn't handle errors that weren't of type Error. In
   a related power move, the mechanism for tracking duplicate errors
   totally failed to handle non-object errors.
instrumentation: nuke special handling for favicon

It cluttered up the instrumentation and is pointless. Gone.
errors: request parameters go in request_params

The agent was previously storing them directly on the params object,
which would have prevented them from showing up in RPM. Always a good
idea to doublecheck this stuff against another agent (in this case, the
Python agent).
collector: simplify remote method invocation

Also rewrote tests to have 1 expectation per clause, with less mocking
and more stubbing.
code: more hidden class optimization
errors: merge correctly on failed collector run

Before, the error tracer was trying to merge the errors object passed to
the DataSender, which is to say the raw JSON expected by the collector.
This was then getting redelivered to the collector, causing it to barf
when unmarhsalling the error traces.

The fix also involved moving responsibility for merging the data into
the agent from the error tracer, and renaming some methods to more
clearly reflect what they were doing. This responsibility should
probably move to the collector connection, so as to keep everything that
deals with collector data formats in one place.
code: hidden class optimization
Updated documentation for new version.
Released 0.10.2