Upgrade to Go 1.4

This change set upgrades Prometheus to use Go 1.4
The other modifications were intended to compare benchmarks between the
old and new versions using golang.org/x/tools/cmd/benchcmp
![screen shot 2014-12-15 at 13 55 55](https://cloud.githubusercontent.com/assets/67471/5436071/56f29dba-8462-11e4-9e06-6961bd358093.png)