Add gitignore

Signed-off-by: Olliver Schinagl <o.schinagl@ultimaker.com>
Transistion from SVN to git

SVN can be quite tedious to work with git is far more versatile and
a bit more modern too. Start using git for the VCS. This additionally
allows to transition from sourceforge, if so desired.

Signed-off-by: Olliver Schinagl <o.schinagl@ultimaker.com>