replace sprintfs with +

As discussed here http://stackoverflow.com/questions/19101419/go-golang-formatfloat-convert-float-number-to-string#comment28244661_19101700 and http://grokbase.com/t/gg/golang-nuts/141v598t2c/go-nuts-string-concatenation-performance using + to concatenate strings is faster than using sprintf.

Local benchmarks show the same thing

    BenchmarkSprintf                     5000000  353 ns/op
    BenchmarkAdd                       50000000  61.9 ns/op
    BenchmarkSprintfWithInt          5000000  339 ns/op
    BenchmarkAddWithInt            20000000  120 ns/op
    BenchmarkSprintfWithfloat64   2000000  823 ns/op
    BenchmarkAddWithfloat64       5000000  381 ns/op