Free the returned value from leveldb_get

This fixes #2
Free the error string from LevelDB

In case of error, LevelDB gives us a malloc'ed string as error
message. Telling the CLR that it's a string doesn't allow us to free
the memory, so change it to IntPtr and free the memory after
marshaling it into a managed string.
Merge pull request #4 from carlosmn/free

Free the LevelDB-allocated strings