Add mocha and gulp test tasks

[kjin]

I've added mocha integration into gulp. I'll describe the tasks in full on the wiki, but what's new here is:

• gulp test transpiles all code (src and test) and runs all tests.
• gulp test.single --file $FILE transpiles all of src as well as $FILE, and runs the transpiled form of $FILE as a mocha test in the same process. $FILE should be a TypeScript file.

The purpose of the latter file is to make it super easy to debug a TypeScript mocha test file in VS Code. Given the following configuration:

{
  "type": "node",
  "request": "launch",
  "name": "Launch current file as Mocha test",
  "program": "${workspaceRoot}/node_modules/gulp/bin/gulp.js",
  "runtimeExecutable": "/Users/kelvinjin/Documents/node-http2/node", // You'll have to provide your own here; VS Code doesn't read env vars in its launch config
  "args": ["test.single", "--file", "${file}"]
}

This makes it so that when VS Code is told to "Start Debugging" (F5), it will automatically take the currently open file, transpile it, and run it as a Mocha test suite.

[nicolasnoble]

LGTM; feel free to merge this once you're happy with it.