add preferGlobal option

[tonylukasavage]

grunt-titanium currently always uses the titanium binary that it installs as a dependency. Unfortunately, the titanium CLI has a terrible record with backwards compatibility and almost always requires the same version be used with the SDK and CLI. We should add a preferGlobal option that will tell grunt-titanium to use the globally installed titanium CLI if it is present, rather than the one installed as a dependency. I believe this might be the root of the issue in #23.