Refactor the model accesors

Refactor the accesors to behave like methods instead of properties.
This means all accesors will be prepended with the string
"get_" and will include a set of parenthesis.

Improve the ModeAttribute class to use the method's
short_description. This commit also adds support for a
new method .help_text attribute has been added.

Move accessors to their own module, named "methods.py".

Remove the PropertyHelper class as the accessors no longer
need it.

Signed-off-by: Roberto Rosario <Roberto.Rosario@mayan-edms.com>

mayan/apps/metadata/classes.py

@@ -1,17 +1,5 @@
 from __future__ import unicode_literals
 
-from mayan.apps.common.classes import PropertyHelper
-
-
-class DocumentMetadataHelper(PropertyHelper):
-    @staticmethod
-    @property
-    def constructor(*args, **kwargs):
-        return DocumentMetadataHelper(*args, **kwargs)
-
-    def get_result(self, name):
-        return self.instance.metadata.get(metadata_type__name=name).value
-
 
 class MetadataLookup(object):
     _registry = []