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.

Closes #877
Why not use the `replace` technique employed in AngularJS core:

```js
var startSymbol = $interpolate.startSymbol(),
        endSymbol = $interpolate.endSymbol();

return template.replace(/\{\{/g, startSymbol).replace(/}}/g, endSymbol);
```