Slider fixes, mainly related to Observable.

1. Use notifyCurrentValue() so that Slider computes correct value on creation
    when the user doesn't specify value, and also doesn't specify min, max, or step.

2. Don't call this.notifyCurrentValue("value") after setting this.value; it's meaningless.

3. Remove some unneeded code from startup().

4. Preserve <input> control specified by application, so that back/forward buttons work.

5. Enable Slider tests.

See ibm-js/decor#8.  Fixes #199, refs #227.
Slider.html sample fixes.

1. put <input> control in test file so that back/forward buttons and form refresh work.

2. Use observe() on slider value, so that it reports programmatic modifications
   in addition to changes initiated by the user.

3. Fix "Set" buttons (on chrome 36, IE).  They need to be type=button.

Slider-dual.html and Slider-single.html still need to be updated.

Fixes #227.