JRuby support

    $ irb
    2.1.1 :001 > require 'rbnacl/libsodium'
     => true
    2.1.1 :002 > RbNaCl::PrivateKey.generate
     => #<RbNaCl::Boxes::Curve25519XSalsa20Poly1305::PrivateKey:491a9830>
    2.1.1 :003 > RUBY_DESCRIPTION
     => "ruby 2.1.1p76 (2014-02-24 revision 45161) [x86_64-darwin13.0]"
    2.1.1 :004 >

    $ irb
    jruby-1.7.12 :001 > require 'rbnacl/libsodium'
     => true
    jruby-1.7.12 :002 > RbNaCl::PrivateKey.generate
     => #<RbNaCl::Boxes::Curve25519XSalsa20Poly1305::PrivateKey:48bd593a>
    jruby-1.7.12 :003 > RUBY_DESCRIPTION
     => "jruby 1.7.12 (1.9.3p392) 2014-04-15 643e292 on Java HotSpot(TM) 64-Bit Server VM 1.7.0_55-b13 [darwin-x86_64]"
    jruby-1.7.12 :004 >