Unbroke the worst for Chrome 32/33 (see #37)

Latest versions of Chrome bring a few new capability checks that block devtools execution, parts of the Devtools not booting up when `Agent.enabled` does not call back and (again) a couple of breaking changes to how profiling works. The worst parts are fixed in here, it's possible to take heap snapshots and CPU profiles again, network tab and (as far as I tested) step-by-step debugging seems to be fine again.

Not fixed in here:
* Source maps now require a properly initialized PageAgent with a main frame, the code in devtools is missing a null-check imo unless coupling PageAgent and NetworkAgent was on purpose
* Flame charts are broken since the CPU profile object's schema still seems to be unstable
* Record heap allocations is broken, will need to reverse engineer from scratch how it works now