Update import_logs.py

Added support to Icecast2 HTTP logs format. NCSA_EXTENDED + SessionTime

*SessionTime is expresed on seconds

I have request this feature a few months ago into this ticket:
http://dev.piwik.org/trac/ticket/3233

Now after some research I have integrated the change, will be great if can merge this into the official release.

I have tested with one logs file with more than

501600 lines parsed, 499714 lines recorded, 213 records/sec (avg), 378 records/sec (current)

Here one screenshot how the stats widget look, after process this log file:

![image](https://f.cloud.github.com/assets/1087447/423589/d251f1b8-ad63-11e2-81bc-39958d732587.png)