Add ObserverEffect and Nil* metrics.

See the comment on ObserverEffect in metrics.go for more.

Note that this changes the signature of New* for all metrics to return the interface type.  I remember a discussion on the mailing list long ago that convinced me to change the return value of those functions to a concrete type but I can't recall why and this is a good reason to change it back.
ETOOCLEVER so s/ObserverEffect/UseNilMetrics/.
Test case for #21.
Fix #21.
stathat support
example has stathat now
ready for pr
update email address
(#22) Refactor StatHat support into its own package.

I would rather not force every go-metrics user to link StatHat's Go client unless they explicitly ask to.
(#22) Document StatHat support.
Resolve naming collision in output functions.

Closes #23.
Merge branch 'observer-effect'

Conflicts:
	ewma.go
Return interfaces from NewRegistry, too.
Remove dead code.
Fix bug in RegisterDebugGCStats()