gofmt + goimports
strftime: exercise different times in Perl tests

This looks like it was the original intent.  --test.short will break
after just one time.
strftime: break build into linux/non-linux

And change the tests to follow.

In particular, strftime on OS X will yield slightly different results in
the face of invalid format specifiers.

Further, once you change a platform to use the Pure version, the
existing Perl tests will only complain about the fact that you are not
using the system version.
strftime: faster StrftimePure

With the new benchmarks, here's the old implementation (on my '13 MBP
Retina, 2.3GHz i7 4-core):
BenchmarkStrftimePure     100000  14621 ns/op  4808 B/op   22 allocs/op

And the new:
BenchmarkStrftimePure     500000  3230 ns/op    304 B/op    4 allocs/op

On the same machine, within VMWare, the Linux benchmarks:

BenchmarkStrftime         500000  3815 ns/op    393 B/op    7 allocs/op
BenchmarkStrftimePure     500000  3141 ns/op    304 B/op    4 allocs/op
Merge pull request #9 from fastly/jed/dev

Miscellaneous changes:

* gofmt everywhere
* Juggle import order for aesthetics
* Avoid some strftime tests under darwin
* Speed up pure-Go strftime implementation