v3.8.0

• Updates expected compiler versions.
• Removes dependencies upon png, zlib and qrencode.
• Backports electrum mnemonic support.
• Adopts boost 1.7.2 as minimum depenency.
• Adopts icu 55.2 as minimum optional dependency.
• Renames library from libbitcoin to libbitcoin-system.
• Various build alterations, fixes, etc.