yahns 1.5.0 - initial OpenSSL support and bugfixes

This release adds basic OpenSSL support for HTTPS connections.

Users must supply a OpenSSL::SSL::SSLContext object which yahns will use
as-is.  yahns will only support HTTPS on Ruby 2.1 and later, as we rely
on "exception: false" in the read_nonblock and write_nonblock methods in
OpenSSL::SSL::SSLSocket.

See the Ruby standard library documentation on how to configure
OpenSSL::SSL::SSLContext objects to pass to the yahns "listen" directive
Editing the yahns config file to use OpenSSL goes something like this:

    require 'openssl' # we will not do this for the user, even
    ctx = OpenSSL::SSL::SSLContext.new
    # user must configure ctx here...

    listen 443, ssl_ctx: ctx

Note: yahns developers are not responsible for bugs in OpenSSL itself
or misconfigured SSLContext objects created by users.  However, our
support of OpenSSL sockets is barely-tested and likely buggy, too.

Furthermore, the "sendfile" (or "kgio-sendfile") gem is no longer a
required dependency.  It is currently impossible to use zero-copy
system calls with TLS sockets.

There are also minor cleanups and a bugfix to ensure body#close is
called for folks using body#to_path where `body' is the Rack
response body.  This bug affected logging using the 'clogger' gem
when serving static files.

Shortlog of changes since 1.4.0

      save around 1500 bytes of memory on x86-64
      http_response: remove arg for Array#join
      remove unused client_max_header_size config
      config: use literal symbol array for now
      http_response: reduce constants for 100 responses
      favor Array#map! for freshly-split arrays
      sendfile_compat: remove dependency on pread
      extras/autoindex: simplify checking non-.gz
      Rakefile: kill more useless gsub use
      initial cut at OpenSSL support
      test/test_ssl: skip test if SSL on older Rubies
      wbuf_common: close body proxies on sendfile abort
      bump published Ruby version requirement to 2.0
      make sendfile an optional dependency
      openssl_client: ignore SSL_accept errors during negotiation

Disclaimer: the yahns project does not and will never endorse
any commercial entities, including certificate authorities.

Shpx Nhgubevgl.