Fix updater

Practically rewrote the updater, since the FEX forwards the download request to Github latest master. 
Also, the folder structure is packed into an additional folder, the MATLAB Search Path 	`/matlab2tikz-matlab2tikz-76c2ac0` (see [FEX](http://uk.mathworks.com/matlabcentral/fileexchange/22022-matlab2tikz-matlab2tikz)). If we were to remove it from FEX, we could simplify things a bit.
Maybe we should rename `status` to something more verbose, e.g. `isUpdateAvailable`?