* Fix build on ARM

* PBKDF2-HMAC-SHA256 and -SHA512 functions
* SNI support in SslStream
* A major rewrite of SslStream internals
    * read and write are now invoked directly on the wrapped stream with no
      intermediary buffering via a custom BIO.
    * NonblockingSslStream is deprecated in favor of SslStream::{ssl_read,
      ssl_write}
    * connect_generic and accept_generic are now equivalent to connect and
      accept
    * A new error type, ssl::Error, is returned from ssl_read and ssl_write,
      and will be the new return type of all other methods in the 0.8 release