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.
Then, I'll proceed with the cleanup: 
check against the new release folders, if those folders contain files that are no longer in the newer release, get rid (will hence delete any custom additions by the user). Same applies to folder no longer in use.

I will warn that the folder structure and its content will be reset to the newest release.