Replace 'only_new' with 'exclude'

The idea is that, rather than having one boolean choice, we can
define a number of string values, such that we can also (for instance)
only predict 'old' links (for e.g. anomaly detection).

If we're directly using a predictor (say, CommonNeighbours), this means one
should change a line like this:

    c = CommonNeighbours(G, only_new=True)

to:

    c = CommonNeighbours(G, excluded=G.edges())
Merge branch 'rework-only-new'

Conflicts:
	linkpred/predictors/base.py
Make EvaluationSheet work with values of n > 1
Add convenience method for when universe is unknown
Handle steps for CachePredictionListener
Make interpolate work for both lists and numpy arrays
Update and add tests for evaluation
Fix bug where LinkPred.excluded was not treated as property
Set Y axis to range between 0 and 1