use the substraction overloaded operator to call the diff between parsers/topologies

[cl4u2]

Example usage:

>>> from netdiff import OlsrParser
>>> url = 'telnet://127.0.0.1:9090/'
>>> p = OlsrParser(url)
>>> q = OlsrParser(url)
>>> p - q
{'removed': [], 'added': []}

[landscape-bot]

Code quality remained the same when pulling 49543a0 on cl4u2:diffit into 94a3a5e on ninuxorg:master.

[coveralls]

Changes Unknown when pulling 49543a0 on cl4u2:diffit into * on ninuxorg:master*.

[coveralls]

Changes Unknown when pulling 49543a0 on cl4u2:diffit into * on ninuxorg:master*.

[landscape-bot]

Code quality remained the same when pulling a435be5 on cl4u2:diffit into 94a3a5e on ninuxorg:master.