add eip-160 preamble and references

EIPS/eip-160.md

@@ -1,8 +1,21 @@
-EDITOR NOTE: below is a copy of the EIP 160 https://github.com/ethereum/EIPs/issues/160#issue-184230277 raw text fetched on 2017-07-17.
+```
+EIP: 160
+Title: EXP cost increase
+Author: Vitalik Buterin
+Type: Standard Track
+Category: Core
+Status: Final
+Created: 2016-10-20
+```
 
 ### Specification
 
 If `block.number >= FORK_BLKNUM`, increase the gas cost of EXP from 10 + 10 per byte in the exponent to 10 + 50 per byte in the exponent.
+
 ### Rationale
 
 Benchmarks suggest that EXP is currently underpriced by a factor of about 4-8.
+
+### References
+
+1. EIP-160 issue and discussion: https://github.com/ethereum/EIPs/issues/160