python-netsnmpagent bugfix for trailing dots in table strings

I have just pushed two fixes that should be of interest to all python-netsnmpagent users:

• netsnmpagent: Make Table’s value() method regard string lengths
• netsnmpagent: Drop special string handling in Table’s init()/setRowCell()

These changes fix the issue with trailing dots in table strings. I’ve so far only pushed them to master. I’d like to get some feedback before pushing these to the 0.5 stable branch, so please check them out :)