Bug 903913 - [Flatfish]Need script to detect device type for reuse in shared folder

https://bugzilla.mozilla.org/show_bug.cgi?id=903913

This patch has added script under shared folder, mainly do below things.
1. detect current screen layout type (tiny/small/medium/large, breakpoints are referred to the design of bootstrap)
2. allow to add/remove mediaquery watcher