Bootstrap 2 to 3, vendor cleanup, LESS refactoring

### Here is the summary of changes:

- Compile only `bokeh.less` which in turn compiles all its "imports" such as `continuum.less`, `flexbox.less`, `widgets.less`. Results in cleaner compiled output
- Updated `modal.js` vendor path in the new Bootstrap 3 folder
- Removed `concat` task from Grunt. All css files are compiled and concatenated into bokeh.css via LESS compiler
- Removed old bootstrap-3.1.1 and bootstrap-2.0.4 folder, it's now simply bootstrap (which is v3.1.1)
- Bokeh toolbar class has been changed for consistency from:
```
<div class='button_bar btn-group'/>
```
to
```
<div class='bk-toolbar'/>
```