feat(rzSliderOptions): Change rzSliderOptions to use expression binding

[rabbitfang]

Switch from using two-way binding for rzSliderOptions to using expression binding. Using expression
binding enables more complex use cases such as using the return value of a function as an options
object.

Closes #265.

[rabbitfang]

If more detailed tests are needed, I can add them. I wasn't quite sure what I should test. I added the optionsExpression option for the test helper for setting the expression that is used for directive (defaults to slider.options).

[rabbitfang]

Example JSfiddle for why this is useful: http://jsfiddle.net/xd2zggvb/

[codecov-io]

Current coverage is 100.00%

Merging #266 into master will not affect coverage as of 9dc5f25

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[ValentinH]

I have to admit that at first I didn't really like it and it feels weird to me, but it works without creating breaking changes (as far as I have tested for now).
I'm just curious of where have you seen such a way to pass parameters to a directive? I was thinking that expression bindings where only good for calling callbacks from a directive...

[rabbitfang]

I personally use expression binding when there is no need for the directive to make modifications to a value (and where attribute binding wouldn't make sense). Expression binding allows the greatest freedom for the consuming developer. One common example for when AngularJS itself uses expression binding over two-way binding is the ngShow, ngHide and ngIf directives. By using expression binding there, I can use something such as ng-if="!shouldHide". It also allows filters to be in the parameter.

For rzslider, I use the return value of a function that dynamically generates the slider's configuration based on various parameters that it pulls from various locations. Even though the $watch uses angular.equals comparison to check for updates, the two way binding only checks for identity. If the function returns a new object on every call, a infinite digest loop is created.

[ValentinH]

Well, looks good to me! Thanks :)