implement X-Mining-Hashrate header

remove midstate extension, seems only used in sha256/scrypt

and prepare noncerange, need a pool which supports that to finish...
Include windows curl and openssl prebuilt libs

Curl 7.35 without SSH2
OpenSSL 1.0.1e
ZLib 1.2.8

built with https://github.com/peters/curl-for-windows
stats: compute work difficulty from target