added entangling_power function
add entangling_power to __init__
unit tests for entangling_power
added reference for entangling_power
added import of qutip.qip.gates.swap in qutip.entropy for use in entangling_power
increased entangling_power test tolerance
added import of future division for py2 compatibility, and reduced tolerance again
Merge pull request #144 from jrjohansson/add-entangling-power-function

Add entangling power function