support for single line in l2vpn vpls; unittests and other fixes

from the list; comments inline
"validation of the input for the data ( thank you for pointing it was not done, I would have missed it :p )"
done. (both inside confiuguration.file and nlri.l2vpn)

adding support for the "single line format ( which is very important for people using the API )"
i think it works (at least no problew with juniper (was using cfg file which ibgp4-l2vpn-singleline.txt))
feel free if you want to somehow change the names etc

adding a test suite for encoding and decoding
done (dev/unittest/l2vpn-tests.py)

the function nlri returns a composed string which I am not sure this is correct ...
renamed that function. actually this is the tricky part. coz i dont know where it used aside from output in syslog
(thru the __call__ and __str__). if it possible, could you point me in code, where else it's used (so i'd be able to fix it)