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
Basically, there is a trade-off:
Angular hard-codes `{{` and `}}` as the "default" interpolation symbols (which is pretty reasonable. If we choose to go this way (having hard-coded default interpolation symbols), we can have a RegExp and use replace.
In the `split/join` approach, we can abstract the default symbols in `$mdConstant`, but since `replaceInterpolationSymbols()` does not know what the symbols are, it is more difficult to create a RegExp (and use `replace`), since you'd have to add provision for esaping special RegExp charactes etc. Using `split/join` we don't care what the symbols are.

--
**Notes:**

- According to [this SO answer][1] (and the accompanying [jsperf][2]), using `split/join` is 20% faster on Chrome, but 60% slower on FF and IE.
- Since we are anyway hard-coding the symbols in the templates, there might be little benefit in abstracting them into `$mdConstant` just for `replaceInterpolationSymbols()`.

--
So, basically, if you prefer it is better to use Angular's approach, I can remove the constant from `$mdConstant` and use `replace` instead of `split/join`. I don't have a strong opinion on which would be better (and probably it won't make any noticable difference either).

[1]: http://stackoverflow.com/questions/1144783/replacing-all-occurrences-of-a-string-in-javascript#answer-1145525
[2]: http://jsperf.com/replace-all-vs-split-join