* encryptSecretKey:
  Implement encryptSecretKey

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.

Closes #79

Provide more guidance for understanding the examples.

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.

Closes #74

In case anyone wants to check it, I guess?

Closes #12

add travis build status to readme

Closes #33

Closes #35

Closes #61

While this message may be more helpful, it will break some cases, such
as when there are multiple ciphers that could be used and we can just
skip CAST5 and move on.  Return NULL when CAST5 unsupported, just like
for other unsupported ciphers.

Mcrypt is deprecated, so use OpenSSL when we can, mcrypt when we can't.

It seems only 2.0.8 is broken

They're very old, and 5.6 is available in Debian stable and oldstable,
so drop support.

Otherwise it returns false, we try to unpack that, and generally bad
things happen.

add suggestion to composer

add php 7.1 7.2 to travis

[doc] change links to https

https://github.com/phpseclib/phpseclib/issues/1113

Add compatibility with phpseclib 2.0.3 - 2.0.4