Merge pull request #43 from euroelessar/master

Break API, break more!
Fixed singed/unsigned warning
v2.22.2.0
Connection refused error message cleanup
Added leveldb checksum function
Return error for CAS write if ->checksum() is not supported
Use __unused for unused arguments
Added offset and append write support to leveldb backend.
leveldb write debug. Free error string if it was set by previous read
v2.22.3.0
Guard leveldb's append/offset writes, since they are actually read-modify-write cycles
v2.22.3.1
Fixed bulk_read for empty request