Refactor perfdata strings

[martialblog]

• Moved util.go into type.go since 'util' is too generic a name
• Made formatting function private, since they should not be used outside the module
• Refactored tests to be table driven
• Less restrictive character set for labels
  • According to the nagios development guides on performance data: "label can contain any characters except the equals sign or single quote (')"

Fixes #48

Also:

• Add benchmark for perfdata
  • The perfdata code does a lot of string operations, we might want to optimize it in the future
  • Example usage go test -run=. -bench=. -count=12 ./perfdata/... | tee my.benchmark

The benchmark for the refactor.

Current master:

BenchmarkPerfdataString-8   	  822433	      1288 ns/op	     192 B/op	      13 allocs/op
BenchmarkPerfdataString-8   	  864208	      1293 ns/op	     192 B/op	      13 allocs/op
BenchmarkPerfdataString-8   	  827318	      1305 ns/op	     193 B/op	      13 allocs/op

Refactor:

BenchmarkPerfdataString-8   	 1394638	       862.3 ns/op	     160 B/op	      11 allocs/op
BenchmarkPerfdataString-8   	 1378656	       868.1 ns/op	     160 B/op	      11 allocs/op
BenchmarkPerfdataString-8   	 1363088	       880.1 ns/op	     160 B/op	      11 allocs/op

[RincewindsHat]

Looks sane