add back off delay to qblast

This pull request is in reference to http://permalink.gmane.org/gmane.comp.python.bio.general/7522 
"query upper limit for NCBIWWW.qblast?"

Previous delay was a constant 3 sec. Now the delay starts at 2sec and increases as follows: delay += .5*delay to a max of 120sec.
The approx. delay and total time is as follows;
(delay, total)(2, 2)
(delay, total)(3, 5)
(delay, total)(4, 9)
(delay, total)(6, 16)
(delay, total)(10, 26)
(delay, total)(15, 41)
(delay, total)(22, 64)
(delay, total)(34, 98)
(delay, total)(51, 149)
(delay, total)(76, 226)
(delay, total)(115, 341)
(delay, total)(120, 461)