Reference state can now be set to one of IIR, ASHRAE, NBP or RESET.  Still need to allow for default reference state values to agree with REFPROP.

Docs updated.

Still need to implement setting state based on numerical values.

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Docs for factory function

Signed-off-by: Ian Bell <ian.h.bell@gmail.com>
Merge branch 'master' of https://github.com/coolprop/coolprop