Added report when program is complete

When dependency_graph.py finishes drawing the graph, a report is printed
to the command line showing how many nodes were drawn and how long it
took.
Added specific type exclusion

Accessed using the -e flag, this allows the user to specify types to
exclude from the graph.

For example, when a link is specified to a node outside the given
dataset, a node of type "Unknown" is created. These can be excluded
from the graph using the -e flag.
Updated README to reflect -e and -x flags' existence