Added taxonomy field. Added ed_category sample vocabulary.

Added a field to be used for category taxonomy named ed_field_category.
This field is relying on ed_category vocabulary being present (this will
be created during installation and removed during uninstall process).
Added a smaple array with taxonomy information for ed_category
vocabulary. The general idea might stay the same, though the information
might be structured differently or the source of data might change.
These methods should probably be moved into a standalone include file.
This change required uninstall/install procedure to be ran.
Added category field for ed_news node.

A category field instance will be added to ed_news node. It is a
required field, thus selecting at least one value is required. Using a
default widget with checkboxes for now (an unlimited amount of
categories can be selected). In furture a better and more powerful
widget should be used, as of the moment term_reference_tree widget might
be considered as it has a collapsible tree-like structure and has some
additional configuration options that might be useful. The change
required uninstall/install procudure.