fix(tooltip): remove child scope requirement

Supersedes #1816. I realized I could resolve a few issues while refactoring the scope.
I intend to merge this first thing tomorrow.

`tt_` scope variables are now in the `ttScope` which is specific to each
tooltip directive (multiple tooltip directives can now run on the same
element).

This allows tooltips to be used alongside any directive that may or
may not require different scope types.

Closes #1269
Fixes #2320
Fixes #2203