refactor(): add -gt- attrs, switch back to sm/md/lg

BREAKING CHANGE: the -phone, -tablet, -pc, and -tablet-landscape
attribute suffixes have been removed and replaced with -sm, -md, and -lg
attributes.

* hide-sm means hide only on small devices (phones).
* hide-md means hide only on medium devices (tablets)
* hide-lg means hide only on large devices (rotated tablets).

Additionally, three new attribute suffixes have been provided for more flexibility:

* hide-gt-sm means hide on devices with size greater than small
 (bigger than phones).
* hide-gt-md means hide on devices with size greater than medium
 (bigger than tablets)
* hide-gt-lg means hide on devices with size greater than large
 (bigger than rotated tablets).

See the [layout options
section](http://material.angularjs.org/#/layout/options)
of the website for more up-to-date information.