Refactor delegation to statsd

The problem I'm trying to solve:
The purpose of `rescue` in method missing is to catch bad method calls to `statsd-ruby`. But it also catches all errors IN `statsd-ruby` method (some methods (`#time`) may raise error because they yield a block). These errors should not be handled by `skit-statsd`.

Also:
* changing global `$statsd` variable to local `@statsd` (shouldn't be used outside of `Skit.stats` anyway)
* removing `log_time`: it's already implemented in `statsd-ruby` and is called simply `time`

@spajus