Updates to the "Ethereum Classic Network Description" which is the canonical description of ETC hard forks

[bobsummerwill]

Updates to the "Ethereum Classic Network Description" which is the canonical description of ETC hard forks.

• Added missing entry for EIP-160 (Spurious Dragon for ETC)
• Minor fix to URL for EIP-684.

We could really use proper "Meta" ECIPs to record these early hardforks, but that is a task for another day.

• "Gas Reprice" (ETC's Tangerine Whistle)
• "Die Hard" (ETC's Spurious Dragon and difficulty bomb delay)
• "Gotham" (Fixed monetary policy)
• "Defuse Difficulty Bomb"

[bobsummerwill]

@atoulme @gitr0n1n @IstoraMandiri @meowsbits

[bobsummerwill]

Withdrawn in favor of https://github.com/ethereumclassic/ECIPs/pull/499.