Updated webapp security analysis code to provide more exploitation context

Updated:

* Timing-attack
* Differential analysis 
* Web module mixin

to pick the right payload and provide the necessary context in order to exploit SQL injection vulnerabilities -- among other things.

Also, added an abstract ```#tries``` method (returns 1 by default) to ```Exploit::Remote::Web```, used to signal to ```websploit.rb``` how many times to run the exploit until we get a session.