Updated README, added gulp-coveralls example

[ghost]

Two small documentation changes:

• Changed the file paths in the examples to be less confusing - gulp.src() takes the test files, cover.instrument() takes the source files being covered. The paths used in the examples seem to imply the opposite. It's obvious enough when adding coverage to an existing test runner task, but 0% line coverage, 0% statement coverage, 0% block coverage, 0 SLOC...? #32 suggests I'm not the first to be momentarily confused..
• Added an example for usage with gulp-coveralls. I've since seen there's a good example in the gulpfile.js, but the readme suggests it's not even possible (says json and html are the only supported formats). I only realised how simple it is after seeing Emit raw LCOV too #25 and since I'd guess it's a fairly common use case, maybe deserves its own example?

[dylanb]

Thanks for your contribution