Add the MMAP backend back in, for experimentation purposes

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
MMAP: factorise read and write

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Rename 'format' to 'backend'

We have multiple backends for the same format (e.g. raw, mmap)

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Update CHANGES, release 1.2.1

Signed-off-by: David Scott <dave.scott@eu.citrix.com>
Merge pull request #13 from djs55/mmap

Improvements to go in sync with wiki page