with-logger macro

One can override the default namespace used in logging with a ```with-logger``` macro (or binding  ```*logger-ns*```).

```clojure
(deftest logp-macro-with-logger
  (with-logger "kikka"
    (info "kukka"))
  (is (= ["kikka"
          :info
          nil
          "kukka"]
        (peek @*entries*))))
```

I have a case where I need to log explicitly to different logs, this seems to be nice way to do it.

Added also few tests for the logp and logf -based macro loggings.
 
regards,

Tommi