Run multiple immediates per browser tick

This roughly doubles browser performance. Still far
behind Jasmine however.

#763.
Don't scroll window while running

This roughly doubles performance again. Plus, the
scrolling is plain annoying.
Do use Object.keys

This doubles performance again. Firefox should fix its damn
bug (#757).
Merge pull request #763 from jfirebaugh/browser_perf

mocha browser runner is 10-100x slower than jasmine on equivalent suites