Refactor dwarf_runner.

[isbadawi]

Summary of changes:

• Removed WebbenchThread (apparently not used).
• Factored out OS-specific bits into OsxEnvironment and LinuxEnvironment classes, each providing a provision_browser(browser, url) context manager, which are passed into Benchmark.
• Added a neato cd() context manager on Benchmark, allowing you to write with self.cd():
• Turned run_c_benchmark and run_js_benchmark into generators
• Simplified options parsing a bit; optparse allows you to set defaults, and a %default placeholder in the help text expands to the default value. (Also removed global variables, instead passing options where they need to go).
• Reduced duplication in the environments; specify each environment as a tuple of language, browser, and function taking a benchmark and returning a generator of the results.

n.b. I use dicts for specifying benchmarks and environments where previously lists were used, so order is not necessarily preserved. If this matters to you, you should use collections.OrderedDicts instead.

n.b. I only did cursory testing of this, since e.g. I don't have emscripten installed, and I am on OS X.

[gnuvince]

n.b. I only did cursory testing of this, since e.g. I don't have emscripten installed, and I am on OS X.

Use a lab machine to make sure it works on Linux (you can just test with JavaScript).

[gnuvince]

Besides the small bug and some stylistic mishaps, this looks good.

[isbadawi]

Ah yes, sorry! I think I ended up not testing this since it failed before that because of emscripten. I tested the C path. I've updated the commit.

I tried testing just JavaScript but it looks like I can't; running make fails because of the missing emcc before generating the plain JS html files.