Bug 866532: expose prePath property on URL instances and making URL properties writable

Conflicts:
	lib/sdk/url.js
	test/test-url.js
Bug 866532: Fixing broken tests from the previous merge
Merge pull request #1018 from erikvold/866532v2

Bug 866532: expose prePath property on URL instances and making URL properties writable a=@gozala r=@erikvold