add OpenSSL support for symmetric key encryption

Replaces #259

There are a few other things I'd like to do before this is merged.

1. do the continuous mode tests without continuous mode enabled to ensure that non-continuous mode operates correctly.
2. rename CRYPT_DES_MODE_3CBC and CRYPT_DES_MODE_CBC3 to CRYPT_MODE_3CBC / CRYPT_MODE_CBC3 and test each of those.
3. make it so Crypt_RC2 assumes the key size is the length of the key that was passed to it? I might make this a separate and unrelated PR. I also want to look at the past discussions on the subject because I think this has been previously discussed at https://github.com/phpseclib/phpseclib/pull/124
4. test both internal modes? i'm actually leaning towards not doing this. for one, so long as their are two internal modes it'd almost make more sense to have 2x internal mode engine constants. eg. CRYPT_ENGINE_MODE_INTERNAL_INLINE and CRYPT_ENGINE_MODE_INTERNAL. But with https://github.com/phpseclib/phpseclib/pull/591 we could just remove the second mode all together (see emails to petrich from 4/16/13). Also, altho calling `$this->_setupInlineCrypt()` every time `isValidEngine()` to see if it is setting `$this->use_inline_crypt` to false (which it is in Crypt/Base.php) instead of how it is currently called would result in inefficient memory usage. ie. `isValidEngine()` is called whenever the key is set or the IV. So if you set both it is called twice whereas it only needs to be called once.

With any luck this'll make #458 more testable (ie. I get the impression mcrypt is becoming supported less and less, as the years pass, whereas openssl is becoming more and more supported; for example, doing `sudo apt-get install php5-cli` on Ubuntu yields a PHP install with OpenSSL but not mcrypt; mcrypt is still built into Windows PHP installs whereas OpenSSL is not, but who knows how long that'll continue to be the case).

Also, the following might make for some interesting reading:

https://github.com/terrafrost/phpseclib/blob/f6e0c4b506b0bdb310314aabfca8d0b7244fe8ba/tests/Unit/Crypt/RC2.php#L17