Make darwin/linux Packets methods consistent

v0.5.0

make darwin/linux interfaces consistent

DeviceType String method

v0.4.0

stringify DeviceType

Fixes for darwin

v0.3.0

add GetMessageType

v0.2.0

Changes in this release:

• Added Devices func to list the devices on the system.

v0.1.0

First release!

Currently supports only ALSA.
I have done a simple test with a Novation Launchpad Mini on arch linux, and haven't seen any issues so far.