use more openssl tools (portability)

On at least OSX, I'm getting bad output from the example data:

Public-Key-Pins: max-age=5184000; pin-sha256="HWqQWajyRtiiXL5w0V4Xq5I9jrR8knuwXYGaTqN1vrE=";pin-sha256="3GvMoKbahfHC8K6s8zwhRDNlZ1Z8nvx6fhCRCixQPhk=";

I can also verify by following the recommendations in the upstream spec (http://tools.ietf.org/html/draft-ietf-websec-key-pinning-20#appendix-A) that this tool outputs different results from the example script in the spec, when running on OSX.

Note that this is *after* altering the hpkp-gen script to call "gsha256sum" from the Homebrew-installed "coreutils" package... there isn't a stock "sha256sum" to call.

This changes hpkp-gen to be more like the reference spec, which uses only openssl-supplied tools plus "base64", and thus is likely more portable. Indeed it does work properly on OSX.

The downside is it requires temp files, as both "openssl asn1parse" and "openssl dgst" are not pipe-friendly (the former won't output to a pipe, and the latter won't read from one). I've opted for the very simple "mktemp" to handle this.