Add measure task to measure image metrics

Add a measure task which on the hand provides a way to measure image metrics and on the other hand is a testbed for signaling meta data. As of now the following works:

``` python
import ufo.numpy
from gi.repository import Ufo

pm = Ufo.PluginManager()

reader = pm.get_task('reader')
measure = pm.get_task('measure')

def show_measure(m, a):
    a = ufo.numpy.asarray(a)
    print(a)

measure.connect('result', show_measure)

g = Ufo.TaskGraph()
g.connect_nodes(reader, measure)
sched = Ufo.Scheduler()
sched.run(g)
```