Add support for parity even

[Salabasti]

In usb_serial version 0.1.1 there seems to be no constant for setting the parity of a UsbPort object to 'even'.

I explored the underlying java library and noticed that this option is supported there. (link) I tried using '2' as a constant for setting the parity in UsbPort.setPortParameters() with no success. EDIT: Passing 2 in UsbPort.setParameters(...) worked for me.

Will the option to set the parity to 'even' be available in the near future or is there any workaround for that?

[altera2015]

I've added the 'Even' constant in 0.2.0

[Salabasti]

Using an Integer value of 2 seems also to work for setting the parity argument to 'even'.

[Salabasti]

I've added the 'Even' constant in 0.2.0

Thank you very much!