Fixed errors and added missing function

There is no function getDevices, it's listDevices. The first argument in the callback seems to be `err`, and not `devices`.
Also found the method I *actually* wanted in the source code, `getDevice` by it's id. So I added a short doc for it as well.
Merge pull request #254 from gregersrygg/patch-1

Fixed errors and added missing function for SparkJS