Interrupts aren't open drain by default

[P4rk]

In the init for the MCP23017 the comments say that setting IOCON.ODR (bit 2) to 0 sets the interrupts to be open-drain.
This in fact sets the output to use the active drivers and the IOCON.INTPOL for active high and active low.

Created a PR to update the comments on the init function here #25

Thank you for the library.

[P4rk]

PR merged closing issue.