[changed] Internal variables classSet to classNames

This has been causing a lot of confusion for folks since React has
depricated `classSet`. We felt at the time that changing the require
call to `require('classnames')` withing changing the variable name would
be enough. It hasn't due to the confusion people are seeing.

Fixes react-bootstrap#489

Author: mtscout6

Diff for: docs/examples/.eslintrc

@@ -6,7 +6,7 @@
   "globals": {
     "mountNode",
     "React",
-    "classSet",
+    "classNames",
     "Accordion",
     "Alert",
     "Badge",

Diff for: docs/examples/CollapsableParagraph.js

@@ -20,7 +20,7 @@ const CollapsableParagraph = React.createClass({
     return (
       <div>
         <Button onClick={this.onHandleToggle}>{text} Content</Button>
-        <div ref='panel' className={classSet(styles)}>
+        <div ref='panel' className={classNames(styles)}>
           {this.props.children}
         </div>
       </div>