Bug 937982 Add support for a DELETE request method

I needed to make a DELETE api call and added support for the method. I attempted to write a test, but I don't see a method of getting at the Request METHOD from the test. I duped the HEAD test case and just verified that it's callable.

I'm having a hard time running the tests to verify my test is valid. 

./bin/cfx testcf works well and I get all passed tests. 
./bin/cfx testall fails with the below error even in trunk without my changes.

JavaScript strict warning: chrome://browser/content/urlbarBindings.xml, line 672: reference to undefined property this._value
JavaScript error: chrome://browser/content/urlbarBindings.xml, line 654: aUrl is undefined
^CTraceback (most recent call last):
  File "./bin/cfx", line 33, in <module>
    cuddlefish.run()
  File "/home/rharding/src/firefox-sdk/addon-sdk/python-lib/cuddlefish/__init__.py", line 621, in run
    test_all(env_root, defaults=options.__dict__)
  File "/home/rharding/src/firefox-sdk/addon-sdk/python-lib/cuddlefish/__init__.py", line 393, in test_all
    test_all_testaddons(env_root, defaults)
  File "/home/rharding/src/firefox-sdk/addon-sdk/python-lib/cuddlefish/__init__.py", line 440, in test_all_testaddons
    env_root=env_root)
  File "/home/rharding/src/firefox-sdk/addon-sdk/python-lib/cuddlefish/__init__.py", line 945, in run
    pkgdir=options.pkgdir)
  File "/home/rharding/src/firefox-sdk/addon-sdk/python-lib/cuddlefish/runner.py", line 719, in run_app
    time.sleep(0.05)
KeyboardInterrupt