Add compression option

[abbbi]

Add compression via python lz4 bindings during backup. Ideas:

To support inline compression during backup, using the sparsestream format, the following has to be taken into account:

1. Enhance sparsestream format for version 2, add information about compression method and wether if compression was enabled to metadata header
2. After saving all data to backup stream, dump json information about the data blocks original and compressed size as json array at the end of the backup stream (after the last STOP frame)
3. After the json information has been dumped to the file, add new sparsestream frame type "comp" which is a regular header with offset and size information about the size of the dumped json information as very last entry to the stream file.

During restore:

1. read metadata header and check if compression is enabled
2. if compression is enabled, seek to the very end of the file and read the last frame type, which holds the "comp" information
3. Seek back the size read from the "comp" metadata header
4. Read the json array with hold the original and compressed data size.
5. Use this information for reading the data blocks in compressed size, decompress and write to target file, check if decompressed size matches the original size of the data.

[abbbi]

Feature is implemented in master branch, but needs some testing :)

[abbbi]

Version 0.22 includes --compress option.