WIP - fix(*): make templates work with custom interpolation symbols

Previously, components specifying their own templates (such as Dialog, Toast etc) assumed that the
interpolation start-/endSymbols where `{{` and `}}`, thus breaking in apps using custom symbols.
This commit introduces a new `$mdUtil` method for replacing the "assumed" symbols (`{{`/`}}`) with
the actual ones.

**NOTE:**
This is still work-in-progress.

 ##### Already completed:

* Added `INTERPOLATION_SYMBOLS` property in `$mdConstant` with the default symbols used in
  templates.
* Added `replaceInterpolationSymbols()` method in `$mdUtil` for replacing default symbols
  with actual.
* Added tests for `$mdUtil.replaceInterpolationSymbols()`.
* Updated `$mdToast` to use `replaceInterpolationSymbols()`.
* Added tests for `$mdToast` with custom interpolation symbols.
* Updated `$mdDialog` to use `replaceInterpolationSymbols()`.
* Added tests for `$mdDialog` with custom interpolation symbols.

 ##### Still pending:

* Find out why `mdTextFloatDirective` works as expected (although it seems that is shouldn't).
* Update `mdTextFloatDirective` to use `replaceInterpolationSymbols()`.
* Add tests for `mdTextFloatDirective` with custom interpolation symbols.

Closes #877