Enable strict mode only at function scope level (not at global)

When using ngToast along with other libraries (concatenated in the same Javascript file), `use strict` is set a global level, affecting the file as whole and therefore breaking libs that don't support strict mode (like [D3](https://github.com/mbostock/d3), see issue [#1755](https://github.com/mbostock/d3/issues/1755)).