utlise proxy on systems where Proxy is supported

[capaj]

As recent development caused O.o strawman proposal to be withdrawn, I think it should be only sensible that observe-js would shift from using native O.o to utilising it's own omplementation of O.o in raw JS utilising proxy- something very similar to https://github.com/tvcutsem/harmony-reflect/blob/master/examples/observer.js

This way, dirty checking is really needed only for older browsers without O.o and without proxies.
Anybody feels up for making this PR? I will probably have time to do this in January, not sooner.

[capaj]

I got a little time to play around with observe-js repo and it seems like I can't even run the tests after cloning- I get:

grunt test
>> Tasks directory "../tools/tasks" not found.
Warning: Task "override-chrome-launcher" not found. Use --force to continue.

Aborted due to warnings.