Refile::CustomLogger adds simple logging of requests to Refile::App.

This commit adds custom logger to Refile::App with duration tracking.
In the end Rails log file looks like:
```
Started GET "/attachments/store/316216/original_file.jpg" for 127.0.0.1 at 2014-12-28 15:43:31 +0200
Refile::App: [2014-12-28 15:43:32 +0200] GET "/store/316216/original_file.jpg" 200 225.5ms


Started GET "/attachments/store/316217/original_file.jpg" for 127.0.0.1 at 2014-12-28 15:43:41 +0200
Refile::App: Could not find attachment by id: 316217
Refile::App: [2014-12-28 15:43:41 +0200] GET "/store/316217/original_file.jpg" 404 1.5ms
```

Refile::CustomLogger is simplified Rack::CommonLogger with basic(most useful information)