Add support for default ca-cert files

Hi,

this branch addresses https://github.com/OfflineIMAP/offlineimap/issues/18.

I only tested it on Ubuntu, it works fine there.I collected the other location from a google
search, they may not be correct but there will be a info message (or should it be a warning instead?) if the distro is known but the location is wrong. There is also a message if its a linux distro that is not in the dict DISTRO_TO_CA_CERTFILE. So hopefully people start to contribute the correct location if I messed it up ;)

I added a test too, let me know if I did it right, the code seems to use a custom testrunner that I don't know. Also I would normally use python-mock but it
appears its not used yet in the code so instead of adding it I just added the trivial "MockUI" 
object.

Happy to fix any issues or do it differently, just say so! 

Thanks,
 Michael
This is Linux-centric, not the entire world is Linux. Try a multi-key dictionary with platform.system() as the first key ('Linux' for the data you have right now). Then for Linux you can use linux_distribution()[0] as the second-level key.

For platform.system()=='FreeBSD' please add '/usr/local/share/certs/ca-root-nss.crt' (requires the security/ca_root_nss port to be installed).