Add DomHelper loadStylesheet method and use it where applicable.
Do not set onload/onerror handler to null after handling the callback.
Use DomHelper.prototype.insertInto instead of appending to head manually in loadStylesheet method. The reason for doing this is so that existing code doesn't behave slightly different from previous behaviour in the case of a badly formed HTML page.
Move tests assets into spec/fixtures directory.
Merge branch 'master' into bs-stylesheet-timeout

Conflicts:
	spec/core/domhelper_spec.js
	src/core/domhelper.js
Merge pull request #147 from typekit/bs-stylesheet-timeout

Add DomHelper loadStylesheet method and use it where applicable.