Return all values with a single read.

[caternuson]

Currently, each property does its own I2C transaction and returns the items of interest. This means that to get multiple values, like button(s) and joystick, separate I2C transactions must be done for each. This isn't a huge issue, since the I2C transaction is now fairly quick (see #20) and there are only a few total properties for the Nunchuk. However, it might help for some use cases to provide a way to return all of the properties obtained from the singe read.

[caternuson]

Done! Taken care of by #24. Thanks @jfurcean