Adding support for decrypting #encrypt elements. Encrypting form elements should now be completely seamless!

- Fixed bug in tests by using drupal_get_private_key instead of variable_get().

- Disabling encryption methods with dependency errors.

- Added MD5 method for the file key provider.

- Added some advanced_help documentation.

- Added hook_uninstall to remove variables on uninstall.
- Still need to move encryption methods to ctools plugins, and document new API.

- Added a couple default key providers.

- Changing occurrances of drupal_strlen and drupal_substr to strlen and substr, respectively. Not sure what is happening, but the drupal_ wrappers appear to be dealing with encoding incorrectly.
- Finishing up tests.

- Tests for each of the implemented encryption methods. ("Basic" doesn't work currently?)

* This modules provides a simple API for two-way
  encryption.