Show metadata help text form in bootstrap popup

This PR displays the help_text in the metadata forms in a bootstrap popover. See screenshot below.

![massgis_gisdata_eotroads_arc__example_com 2](https://cloud.githubusercontent.com/assets/506078/3232514/daed1cae-f0b4-11e3-9e7c-97372c182c02.png)

In WorldMap, we needed to be able to override this text and make it much longer including html links and formatting as displayed below. This can be accomplished by monkey patching (i.e. in your urls.py) the help_text in your own project like this.

geonode.layers.forms.LayerForm.base_fields['constraints_other'].help_text = "Some monkey patched help text with <a href='http://geonode.org'>html links</a> and <b>other markup</b>"

![massgis_gisdata_eotroads_arc__example_com 3](https://cloud.githubusercontent.com/assets/506078/3232533/032dd352-f0b5-11e3-8216-de171033eed5.png)