#321 changeDefaultPollingPeriod does not work

[Carlos Falcon]

changeDefaultPollingPeriod gives an Exception if the epic library is not installed.

It can be reproduced with:

from taurus.core import TaurusManager

m = TaurusManager()

m.changeDefaultPollingPeriod(1000)

The problem is in _get_plugin_classes() method.

[Carlos Pascual]

Fixed with https://github.com/cpascual/taurus/pull/30