JournalTouch checkout integration release branch

[krugar]

the idea has come up to allow adding books / articles displayed in libmon-php to be added the the cart that is included in an integrated JournalTouch installation (cf bibliocoll/JournalTouch#25).

This kills my original plan to rewrite libmon as a static website generator (or configuration script thereof), as JournalTouch is moving towards sessions for data storage and libmon-php will need to integrate with that.

given that not every libmon-php installation will have JournalTouch integrated, we need to figure out a way to separate three cases:

• no cart, no checkout, no sessions, no change (but session keep-alive?)
• provide cart and a checkout system, but have no JournalTouch installation to integrate with
• add the cart, and integrate with the checkout system of an existing JournalTouch installation

the latter two cases will require libmon-php and JournalTouch to initiate sessions that can be handed back and forth between both (lets assume they're running on the same machine, for now) and a common data format understood by both for display and checkout purposes.

this means the cart-sessiondata thing will need to be a module integrated (or integrate-able) into both projects but dependent on neither.