[feature] Change base class from Entity to object

[hkchekc]

Refer to Issue #389.

As suggested in #389, the features should not have the attribute name. Also, feature should not have type_ and definition attributes. Its string representation should also be different. Therefore, this PR remove the base class entity.

As an result,

• Feature will not have name, type, definition attributes anymore
• The contains method of feature container will not use the feature.name
• create_at and update_at property is kept and moved to feature
• Change the feature string repr to showing data.name and link_type

[coveralls]

Coverage decreased (-0.1%) to 89.097% when pulling 3d6dca5 on hkchekc:no_name into dcb8ddc on G-Node:master.