Added multipart upload, added optional encryption/decryption.

### Summary

* Added new `multipart_upload` command with optional number of threads as well as chunk size arguments;
* Integrated `python-gnupg` package and added optional encryption/decryption of files using GnuPG's RSA algo implementation. 
* Added optional `--encrypt` and `--decrypt` flags for enabling and disabling encryption respectively. The `--encrypt` is available for upload commands, whereas `--decrypt` for retrieval;
* Cleaned up the code a bit;
* Updated `README.md` and added `.gitignore`;

### Testing

Sucessfully uploaded and downloaded multiple files with and without encryption using multipart as well as single chunk upload method. Files were of different sizes ranging from 1KB to 3GB.