[GEOS-5993] Missing ElectronicMailAddress in WFS capabilities

In the WFS capabilities there were missing two important fields that are
present in the XSD. Those fields are ElectronicMailAddress and
DeliveryPoint.

This commit adds those two fields, but disregards the fact that there
can be more than one of both fields. However this patch could be adapted
to create a List or Array of both, so they are more faithful to the XSD.
[GEOS-5993] Added a test in the WFS Capabilities Transformer test for Contact information added to WFS.
[GEOS-5993] Added GUI support for DeliveryPoint and ElectronicMailAddress.
Implemented hashCode and equals methods.
Updated SLD cookbook 3-color gradient to match current behavior
[GEOS-6688] Addition to support more interpolation types.

The nearest neighbor interpolation type was not working because of an issue regarding spelling of the word neighbor. When enforcing one style of spelling we potentially break a lot of existing configurations in GeoServer. That's why it is more convenient to check with startsWith() instead of equalsIngoreCase(). This way all of the existing interpolation methods are working while maintaining compatibility with the differences in spelling.
[GEOS-6736] Allow admin to secure WPS processes based on current user roles