Prefer actual child value to childPath.value when traversing.

Previously we asserted they were the same, but that was not necessarily
true in cases when the childPath was cached by the parent path.
Deindent an unnecessary else branch.
Simplify NodePath#_computeNode.
Switch from Whiskey to Mocha for unit testing.

Closes #20.
Upgrade esprima to v1.0.4 and move esprima/mocha to devDependencies.
Bump NPM version to 0.3.15 in preparation for publishing.
Fix typo.
Merge pull request #19 from amasad/master

Handle more yield expression parent expression types that needs parens.
Temporarily allow failures in Node v0.6 to work around NPM issues.