Allow linking to locations within grains

This works by adding a query parameter `location` to grain URLs
Add message handler to allow setting location in url from child iframe
Fix according to Kenton's code review

* All references of `location` changed to `path`
* Path is now part of the URL, instead of a query parameter
* Fix up origin checking in message handler
* Change message handler to run on Meteor.startup
* Use replaceState instead of pushState for path changes
Change error code to 400 for invalid location being supplied
Merge pull request #85 from jparyani/grain-links

Allow linking to locations within grains