Add U8glib as a dependency to platformio.ini for those who use PlatformIO; this makes compiling even easier as u8glib will be downloaded and included automatically.

Also added the new .pio-related directory to .gitignore, and a few ignores for those who use cmake.

Refer to http://git-scm.com/docs/gitignore
to see why this is appropriate

Move the .gitignore out of the Marlin subdirectory so it applies to
the whole tree, and add some missing patterns.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>