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
As I said, the version is 4 bytes long (32-bit). So by definition, we can't have more than 2^32 different versions, and so obviously 2^32 enumerations are enough. My argument is whether an enumeration type is binary compatible with an unsigned 32-bit integer. Currently we save the version on disk, as a 4byte array, directly casted from an unsigned uint32 (little-endian).
If the enumeration type is not binary compatible with an uint32_t (and as far as I know, there is no such guarantee) , then a translation layer is needed to convert between them.