Purge Sessions Tool

Tool for the STK, it purges sessions. Copied with slight modification from the ACP purge session tool.

My board seems to have issues once in a while that makes me unable to login unless the sessions table is cleared and a tool here is easier than logging into phpMyAdmin all the time. I don't know if others experience this issue or not, but I thought I'd give you the tool so you can include it in future versions of the STK for everyone.