Issue 140: Engineering release of AIT encryption support

AIT now offers an encryption API with some supported implementations, and to which extensions can be added. The default implementation, NullEncrypter, supports the API but performs no encryption/decryption.

For projects which utilize the MGSS KMC SDLS Encryption service, AIT offers a wrapper for that service, KmcSdlsEncrypter.

Encryption is currently limited to support command uplink (TC Frames), though may be expanded for downlink at some future date.

This PR supports a schedule engineering release of the AIT Encryption API for use by SunRise.

General documentation is available here: https://github.com/NASA-AMMOS/AIT-DSN/blob/issue-140/doc/source/encryption.rst

Included is a new command-line tool ait_encrypt, which allows the binary contents of a file to be encrypted and decrypted using available encrypter classes:

usage: ait_encrypt.py [-h] --mode {encrypt,decrypt} --type TYPE --input INPUT 
                      [--output OUTPUT] [--verbose]

Performs encryption or decryption on bytes from a file.

optional arguments:
  -h, --help            show this help message and exit
  --mode {encrypt,decrypt}
                        Mode: 'encrypt', 'decrypt' (default: encrypt)
  --type TYPE           Type of encrypter to use: 'null', 'kmc'; or full class name
                        (default: null)
  --input INPUT         Input filename, required. (default: None)
  --output OUTPUT       Output filename, optional. (default: None)
  --verbose             Print debug messages. (default: False)

Examples:

  $ ait_encrypt --mode encrypt --type null --input input.bin  --output output.bin
  $ ait_encrypt --mode decrypt --type null --input output.bin --output restored.bin

Fixes #140

NASA-AMMOS / AIT-DSN

Issue 140: Engineering release of AIT encryption support #142