Make scp task chainable\dependable

Added callback to options object to be able to chain tasks in gulp.taks via callback
See https://github.com/gulpjs/gulp/blob/master/docs/API.md#async-task-support

Since gulp-scp does not support streaming, returning stream (as per API doc) does not work.
gulp-scp could be refactored to support streams, or simply have callback(which I did).