Fix #7108: btn group styling

adjust description & give source for nested `.btn-group`s example

Greater standardization of sizing terminology

In class names:
* large => lg
* small => sm
* mini => xs ("Extra small")

In screen size categories:
* Tiny => Extra small

fix #9051

Change value with variables for btn-large, btn-small and pagination

organizes the Gruntfile a bit and makes better use of Grunt conventions 

This pull request organizes the Gruntfile, makes better use of Grunt conventions and placates my OCD.

Changes in the Gruntfile:

* Use two spaces for consistency with the rest of the codebase.
* Tasks are now organize based on task type. since lodash templates are used across tasks/targets, this makes it easier to follow.
* Use `src-dest` files format when only src-dest pairing is necessary. This saves a line over the files-object format, but more importantly it allows use the src or dest in lodash templates (e.g. `<%= concat.bootstrap.src %>`), which isn't possible in the files-object or files-array formats.
* Make the explicit order of the javascript files more obvious in concat task , maybe just a tiny bit more usable and easier to customize - but really just OCD
* Use `<%= pkg.name %>` variable for dest CSS, both for consistency with how javascript targets are defined, and to make it just little easier for devs to customize bootstrap.

ran the build to test, everything seems good.

using @line-height-computed in .navbar-nav link

When a user changed the @line-height-base/@line-height-computed var, the
nav-bar items would not scale appropriately.

Fix Horizontal form docs

Conflicts:
	dist/css/bootstrap.min.css

add nav to media object and thumbnails

Conflicts:
	dist/css/bootstrap.min.css

use separate `form-group`s in horiz form example for greater generality