Addition of "stopDiscover*" methods

[raykamp]

This feature request came from this issue: #9 (comment)

"It seems like there should be a way to have a user request that a discovery attempt should end. If I call "discoverAll", then I can end this attempt by calling "stopDiscoverAll" with the same listener function. If I call "discover", "discoverWithFilter", or "discoverByUuid" then I'm unable to cancel these discovery attempts."