Added new public method  base_object::drop_attrib(id)

Issue 2: Added a base_object::drop_attrib() to allow the safe
removal of an effector from either the pre or post lists. Effectors
are dropped during the cleanup phase of the tick method. drop_attrib()
itself simply queues the provide effector id in a new private
vector dropped_attribs for removal on the next tick. Unit tested and
ready to go.
Corrected a failure to clear the dropped list after use.

Issue @I2 : Upon review, discovered that I failed to clear the
base_object::dropped_attribs list after removing the dropped
effectors. This would have lead to slowdowns over time. Corrected
and unit tested.
Merge pull request #8 from fpstovall/effector_self_remove

Added new public method  base_object::drop_attrib(id)