[Crypto] Add Initial PolarSSL Support

This patch adds PolarSSL support for SS and does not corrupt the compilation with original OpenSSL library. Crypto library can be specified by arguments using `./configure` (default OpenSSL). OpenWrt build script is also added with a PolarSSL variation.

However, since PolarSSL is so light-weight that only supports part of crypto methods: RC4, AES, Blowfish and Camellia, while others are not supported currently. But I think, this patch could still be useful especially for users using embedded systems who care a lot about the file size.

PolarSSL `ss-local` is tested to work with OpenSSL Node.js and libev servers. To ensure the compatibility, more tests need to be done before merging.