gnuPG class: Encrypt data and manipulate keys using gnuPG

This class provides an PHP interface to use the functionality of gnuPG to encrypt and decrypt data and manipulate encryption keys.

The implemented functionality includes:

- GenKey: generate a new key.
- Encrypt: encrypt any content
- Decrypt: decrypt previously encrypted content
- Export: export a key
- Import: import a key block
- ListKeys: list the keys in the key ring
- DeleteKey: delete a key from the key ring
- SignKey: make a signature on key

This class not use temporary files, so the passwords and the sensitive information to be encrypt is never made accessible from the outside of the applications that use this class.