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`.

```
benchmark                                old ns/op      new ns/op     delta
BenchmarkTargetManager-8                 38866          19355         -50.20%
BenchmarkTargetPool-8                    114763         58989         -48.60%
BenchmarkFingerprintLockerParallel-8     747            752           +0.67%
BenchmarkFingerprintLockerSerial-8       39.0           39.1          +0.26%
BenchmarkAppend-8                        2020           2553          +26.39%
BenchmarkFuzz-8                          1185584705     506752314     -57.26%
BenchmarkLabelPairs-8                    753            903           +19.92%

benchmark                                old allocs     new allocs     delta
BenchmarkTargetManager-8                 41             46             +12.20%
BenchmarkTargetPool-8                    682            460            -32.55%
BenchmarkFingerprintLockerParallel-8     0              0              +0.00%
BenchmarkFingerprintLockerSerial-8       0              0              +0.00%
BenchmarkAppend-8                        11             15             +36.36%
BenchmarkFuzz-8                          2269055        1061400        -53.22%
BenchmarkLabelPairs-8                    12             12             +0.00%

benchmark                                old bytes      new bytes     delta
BenchmarkTargetManager-8                 8755           7160          -18.22%
BenchmarkTargetPool-8                    82621          41342         -49.96%
BenchmarkFingerprintLockerParallel-8     0              0             +0.00%
BenchmarkFingerprintLockerSerial-8       0              0             +0.00%
BenchmarkAppend-8                        289            293           +1.38%
BenchmarkFuzz-8                          2307248006     586094916     -74.60%
BenchmarkLabelPairs-8                    316            304           -3.80%
```
Merge pull request #424 from tsenart/go1.4

Upgrade to Go 1.4