Replace position() with offset() in scrollspy.js

/cc @leeaston

Updated BS3 form classes

* Once again no longer applies to single inputs (this is a bit simpler,
but I'm open to new ideas) but the entire set of inputs, labels, and
help text within a particular element. However, the styles are not too
dependent on markup or layout, so they're super flexible.
* Simplified the markup in the validation docs examples to match latest
changes.
* Renamed `.form-state-validation` mixin to `.form-control-validation`
to match naming of prev commit.

* Reorganize forms.less
* Change from attribute selectors to `.form-control`. Few lines, less
specific (meaning easier overrides as `element[type=""]` is more
specific than a class), less intrusive, and more performant.
* Add `.form-group` for basic spacing in vertical forms.
* Remove (unnecessary?) `margin: 0;` from `form` element (as far as I
can tell no browser sets that anyway).

twbs ALL the links

When using navbar heights higher that default, the navbar-toggle button is not centered.

Fiddle demonstrating this issue (compiled with `@navbar-height: 60px;`): http://jsfiddle.net/tagliala/mRVMe/2/


Another fiddle with the proposed fix: http://jsfiddle.net/tagliala/mRVMe/3/