zaproxy: Break points : allow these to specify request and/or response

Right now custom break points apply to _both_ requests and responses.
It would be much better if the user could choose if one applies to a request, a response
or both.
Proposed on https://groups.google.com/d/msg/zaproxy-develop/wR90nefqdG4/XoRDvqMWuHUJ

This should be a relatively straight forward change to make - the relevant code should
all(?) be in: https://code.google.com/p/zaproxy/source/browse/#svn%2Ftrunk%2Fsrc%2Forg%2Fzaproxy%2Fzap%2Fextension%2Fbrk

Original issue reported on code.google.com by psiinon on 2014-07-23 09:34:47