ENH: limits on EpicsSignals and Positioners (issue #25)

Simple-ish fix for limits all around. Recall that the motor record sets lower and upper ctrl limits on the `VAL` field, so special handling isn't really necessary.

One improvement I can suggest is to (somehow) automatically check if limits should be enabled for the `EpicsSignal`. As it is, using it for all EPICS signals causes issues with many other signals (try setting `limits=True` in `EpicsSignal`'s initializer and running the areadetector example).