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
Fixed error in globalphase
Fixed globalphase to use sparse matrix
Fixed resolution of CSIGN in terms of CNOT
Added unit test for circuit resolution
Fixed indentation error
Fixed indentation error
Added import circuit
Fixed globalphase
Added more unit test for circuit resolution
Fixed globalphase
Modified latex function
Changed \-pi to -\pi