Internal implementation of DHKeyExchange

Reusing a bignum (well, unsigned very-long) implementation I had lying
around for years and just cleaned up a bit and brought to C++11 land.

It might not be the most performant implementation, but it shoud be fast
enough for our purposes and will go a long way of removing gcrypt,
nettle, gmp, openssl dependencies when using AppleTLS and WinTLS
(upcoming).
Provide internal ARC4 implementation

Now you can build bittorrent support without without external
libraries, meaning you can skip libnettle, libgmp, libgcrypt, GnuTLS and
OpenSSL on OSX (for now).
Merge branch 'master' into bignum-arc4
bignum.h: Fix compile error
Merge branch 'master' into bignum-arc4
InternalARC4Encryptor: Fix compiler warning and remove no use ctor/dtor
InternalDHKeyExchange: Remove unused dtor