Allow use of custom interpolate symbols.

Remove hardcoded interpolate symbols and replace with dynamic ones.
Fix potential problems with minification.

Without stringifying the injected provider, we'd lose it in the minification process.
Update minified version of library.

I didn't know what minification process was used previously. Went with closure compiler to do this.
Merge pull request #21 from ebrentnelson/main

Fix to work with custom $interpolationProvider