Add block based backup (space/bandwidth efficient)

Currently Freezer checks only ctime and mtime inode information to verify if files are changed or not.
While this approach gives speed (time efficient), it is not bandwidth and storage efficient. Freezer needs to support both librsync and libarchive approach to execute incremental backups and restore.

- Freezerc has to be able to compute changes for blocks of files

- The blocks size needs to be configurable. By default will be 65K

- Signature for files blocks size needs to be generate for level 0 full backups

- The comparation needs to be done against:
    - Available files (signature against local files)
    - Available signatures (signature against signature)
    - Available streams (signature against streams)

- The restore of the files can be done under the following conditions:
    - Downloading valid incremental backups (i.e. level 0 to level n) and apply write
        the differences on the file incrementally, block by block and in stream

- Implementation has to be memory efficient (operations are executed against blocks and not against whole files or whole streams)

- Implementation has to be Disk I/O/space efficient (operations are executed from and to stream of data and not anything like: download files/stream locally and then execute action against local files)

Implementation thoughts:

Methods/Functions:

1) gen_sig_from_file(fs_path, ex_signature=None): Level 0 signature generation from file
    return a new full signature signature data struct (dict)
2) gen_sign_from_stream(read_pipe, ex_signature=None): Level 0 Signature generation from stream
    return a new full signature signature data struct (dict)
3) gen_sign_from_file(read_pipe, ex_signature=ex_signature): Comparation between signature and local file (for backup level > 0)

4) gen_sign_from_stream(read_pipe, ex_signature=ex_signature): Comparation between signatures and blocks of streams (for backup level > 0)

5) patch_from_file(fs_path_soruce, fs_path_dst, read_pipe=None): File patching for restore from files to files (for restore).

6) patch_from_stream(read_pipe, fs_path_dst): File patching for restore from stream to files (for restore)