Add quota feature

Hi Graham,

Same work to next release of burp 1.4 :
This is the new patch to add the quota feature.
The soft_quota and hard_quota are checked on the server side and a warning message is displayed on the client if the hard quota is reached or if the soft quota is exceeded.
The information about the options are added to the man page.
This patch is a draft, i'm not sur that is the best implementation.

Best regards