Allow for the use of PATH_INFO

This allows for the Apache server variable, PATH_INFO to be used.

It's a small edit, but it allows for Klein to work properly in a subdirectory
Also, a few things here:

- You should use a variable name that has meaning, it makes it easier to read. Maybe `$uri`.
- The conditional should be a bit more explicit, it should check for `!empty($uri)`
- You should have new-line separation wrapping the block, so one before the if and after the closing brace