Allow encrypting a decrypted secret key, which is especially useful for
generating a new encrypted secret key. Defaults to AES256, S2K
iter+salt SHA512, always uses s2k_useage 254 with sha1 integrity
protection of the encrypted key material.
Also add an example to parallel keygen.php that generates a key and then
encrypts it with a passphrase.
This commit adds an `example/README.md` file with a little bit of
guidance for running the examples themselves. This is helpful because
the examples all rely on the presence of a `phpseclib` installation
available to the PHP interpreter, and while there is a `composer.json`
file to this effect, none of the examples included the Composer
`autoload.php` file.
This commit makes no modifications to the example code itself, but does
`include_once()` the Composer autoload script so that `phpseclib` loads
and avoids causing a fatal error when a new user attempts to run the
examples to learn how to use the library.
This commit also updates the Travis `before_script` build script, dropping
the `--dev` argument to the `composer install` command. Current versions
of Composer emit a deprecation notice when `--dev` is passed.
I'm going to make this one more generic than one library, and have it
support ciphers based on what libraries are available, so this more
generic name is appropriate.
