gulp API documentation enhancements

* Documented gulp.src() options
* Elaborated on gulp.glob() options
* Documented gulp.watch() events
* removed options from gulp.dest()
* Elaborated on gulp.run() concurrency
* Clarified async tasks
* Fixed spelling errors
* Added link to node-optimist
* Added gulp.env example
* Be consistent when referencing methods in backticks
* Re-worded note about streams