Fix updater

(same as #499)

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.
If users don't use git to customize their m2t, they should develop their own way to handle these things or always call with the switches to disable the updates. 
When they use git, the updater is automatically disabled.