Add `in-view-options` to handle offset and debounce options

As a way to add support for custom debounce times (as noted in #21), this PR propose to add an `in-view-options` to the directive (mimicking `ng-model-options`, new in Angular 1.3) to specify debounce and offset (`in-view-offset` is removed which is a breaking change).

The directive would change the way it works as the default debounce would be 0 (that is, no debounce at all; another breaking change).

A typical usage would be:

```
<any in-view="$inview&&someFunction()" in-view-options="{ offsetTop:100, offsetBottom:0, debounce:100 }"></any>
```

This is a possible alternative to #35 to allow element specific configurable debounce and avoid the usage of a provider for configuration. @urecio thoughts?

**TODO:**

- [x] implement customizable debounce
- [x] remove `in-view-offset` attribute
- [ ] fix tests
- [ ] add tests for debounce option
- [ ] update examples and add specific debounce example
- [ ] update readme