Adding the ability to get node path out of .appiumconfig (for users who ...

Users who want to use this feature modify node_modules/appium/.appiumconfig & add a "node_bin":"/path/to/node" key/val pair.
The NODE_BIN in ~/.instruments.conf is always checked first, then $NODE_BIN and if both of those don't have node configured then it looks in node_modules/appium/.appiumconfig for the node_bin entry & if it exist the code then uses that node, otherwise it falls back to using 'which node' & looking in the standard places to find node (like /usr/local/bin).
The main purpose for this change is for projects using buildout and that might not have node installed globally on the machine and want to use the sandboxed (local) node (even if node is installed globally).  This allows different projects to use different node versions side by side.