ENH: set Brain time index with time value

add .set_time() method
RF: Brain.set_time() finding time point
RF: Brain.set_time() checking range
ENH Brain.set_time(): don't depend on monotonously increasing time axis
OPT: Brain.set_time()

use np.min and np.max
Merge pull request #34 from christianmbrodbeck/time

ENH: set Brain time index with time value