Add UShort_t - needed for NanoAODv12

In NanoAODv12, GenPart_statusFlags is a type of UShort_t. Previously it was a type of Int.

The operation like:

lastCopy = op.select(t.GenPart, lambda p: (p.statusFlags >> 13) & 1)

does not work properly and it gives an error:

    lastCopy = op.select(t.GenPart, lambda p: (p.statusFlags >> 13) & 1)
TypeError: unsupported operand type(s) for >>: 'ObjectProxy' and 'int'

Solution: Add UShort_t into the _integerTypes list in bamboo/treeproxies.py

This PR is related to #110