mstor - Headers metadata support

My apologies for neglecting mstor for so long, but hopefully this will make up for it: headers support in metadata.

The idea is to provide header data without loading the message. Hopefully this will reduce the memory usage without impacting on performance.

Metadata now appears to require a little less than 10% of actual mailbox space - perhaps a little high but we may be able to tweak this a little by only storing headers we are interested in.

Currently the changes are only in CVS - will look toward a 0.9.3 release shortly.