Finalized homework

Big homework almost done.
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Single-threaded, simple, without UI
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Bugs:
1) Command Line parser: probable error with options, that contain hyphen
("-")
2) _stat() function returns 0 even if the file does not exists
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TODO:
1) Implement CFB, OFB block-cypher encryption
2) Refactor code
3) Stabilize and error stable code
4) Finalize UI