Feature pithoschanges

Includes the necessary changes for better integration with Pithos

Noticeable changes:

* Add signature for v2 maps
* Add v0 map hint
* Support map creation from a given object list
* Remove "archip_" prefix from maps and blocks created by archipelago
* Add rename functionality
* Immediately delete writeable objects when removing a map
* Add readonly option when mapping a volume
version is uint32_t on purpose. Version is written in map header, and we define that it should be the first 4 bytes of the header, which translates into a uint32_t. 
If we do what you propose, we must add a translation layer between the  bytes stored in the map header, and the enumeration type and vice versa. Now we can simply cast the bytes in the uint32_t type, and proceed.

Also I thing that setting version =  0 is clear enough, and treating it as integer has its merits