Suggested fix for #97, review of implications required

[nilsvu]

With the prepended slash, minimatch will never actually return an object with negate: true, so it's impossible to ignore files in clean operations. Please review this change and consider implications to the remaining codebase - perhaps even consider refactoring to use the glob processing implementation in vinyl-fs, that also gulp.src uses, if possible?

Fixes #97

[morris]

This looks like it could break a lot of scripts relying on the current behavior. Could you update this to still prepend the slash but somehow parse the negation beforehand? Ideally with a test? Many thanks!