Provide a simpler implementation of function parse-line.

The last three lines of the original version of function `parse-line` are hard to comprehend, IMHO. That's why I send you this pull request and please understand the changed function implementation as a suggestion only.
I also profiled both versions of `parse-line` to see if the code simplification has any impact on the performance. In the following output the original implementation has the name  `orig` and my own version has the name `simplified`. For the profiling I used the very nice [timbre](https://github.com/ptaoussanis/timbre) library. As input to both versions of the function `parse-line`I used the `logline` string from `/lumberjack/test/lumberjack/nginx_test.clj` (line 5). I've got following results (as always please treat profiling results with caution!):
<pre>
2013-Apr-10 20:13:03 +0200 localhost INFO [lumberjack-contrib.core] - Profiling :taoensso.timbre.profiling/Parse-Lines
                                 Name  Calls       Min        Max       MAD      Mean   Time% Time
      :taoensso.timbre.profiling/orig   1000      19μs        6ms      36μs      53μs      49 53ms
:taoensso.timbre.profiling/simplified   1000      19μs        9ms      30μs      46μs      43 46ms
                         [Clock] Time                                                     100 109ms
                       Accounted Time                                                      91 100ms
</pre>
So, it's not much from a performance point of view.