Add index-based constructors to rate helpers.
Export swap-helper constructors with discounting curve.
Export overnight indexes.
Export OIS rate helpers.
Updated SWIG bindings for rate helpers.

Where allowed by the underlying library, helpers can now
use and external discount curve. Missing overloads of the
constructors were also exported.
Fixed arguments passed to CmsLeg in test case.

I'm not sure how this was working previously. It's likely that
SWIG 2.0.x used less strict type checks and allowed conversion
to and from booleans. SWIG 3.0 no longer allows this.