Improved brackting utility for univariate solvers.

Bracketing utility for univariate root solvers now returns a tighter
interval than before. It also allows choosing the search interval
expansion rate, supporting both linear and asymptotically exponential
rates.

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1578428 13f79535-47bb-0310-9956-ffa450edef68