Make sure that we don't destroy the current CONFIG

Its possible to also provide a rebar.config which rebar.config.script is
responsible for mutating. This just makes sure we update the provided
CONFIG values rather than possibly dropping unknown configuration
values.
Improved configure scripts

This provides a better help output and argument parsing for the
'./configure' script. It implements a method for pulling config values
into rebar.config.script's so that we can control the build a bit more.

As an example there's now a '-c' option to build couchjs with cURL
bindings.
Import JavaScript test runner changes

This is the basis for some temporary work to be able to run the
JavaScript test suite as during the merge transition.
Make the JavaScript test runner work

This is currently removing a bunch of stuff that we normally use during
the test suite so that I can get the test runner going. I full intend to
undo these changes in the future.
Don't stop running tests on the first failure