Implement the suggestions that fixes #4

I went through the code and implemented the changes that fixes #4.  I ended up making a second structure for the KeyNames so as to make initialization of the record easier.  This update allows someone to write a handler such as:

```
func MyLogFmtHandler(h log.Handler) log.Handler {
	return log.FuncHandler(func(r *log.Record) error {
		r.KeyNames.Msg = "log_message"
		r.KeyNames.Lvl = "log_level"
		r.KeyNames.Time = "log_timestamp"

		return h.Log(r)
	})
}
```
and then invoke it by doing something like:
```
logger.SetHandler(MyLogFmtHandler(log.StreamHandler(os.Stdout, log.LogfmtFormat())))
```

I did some benchmarking on this new version versus the old version.  I found that the `BenchmarkJsonNoCtx` benchmark got significantly faster:  4754 ns/op --> 1616 ns/op.

```
PASS
BenchmarkStreamNoCtx         500000  3856 ns/op
BenchmarkDiscard           10000000   170 ns/op
BenchmarkLinenum            2000000   782 ns/op
BenchmarkLogfmtNoCtx         500000  3530 ns/op
BenchmarkJsonNoCtx           500000  4754 ns/op
BenchmarkMultiLevelFilter  10000000   198 ns/op
ok  	github.com/inconshreveable/log15	12.641s
```
versus
```
PASS
BenchmarkStreamNoCtx         500000  3835 ns/op
BenchmarkDiscard           10000000   193 ns/op
BenchmarkLinenum            2000000   803 ns/op
BenchmarkLogfmtNoCtx         500000  3525 ns/op
BenchmarkJsonNoCtx          1000000  1616 ns/op
BenchmarkMultiLevelFilter  10000000   220 ns/op
ok  	github.com/gtrevg/log15	12.375s
```