Merge pull request #506 from melissalinkert/memoizer-fixes

Memoizer: use Location instead of File to represent 'id'
merge develop -D org -I include -E exclude: PR 512 (xsd-fu: Extend C++ support to generate headers and include guards)
merge develop -D org -I include -E exclude: PR 518 (ScanR: clear the pixel buffer when an invalid TIFF is found (rebased onto develop))