add darken splash background functionality, closes #183

[sukritchhabra]

• Adding a slider to splash module that allows darkening of background.
• Also changing gulp-uglify version to 1.5.4 in package.json.

[thomaspark]

Hey @sukritchhabra, this looks and works great! What if we changed the max opacity from 0.5 to 1.0 to give it the full range?

[sukritchhabra]

I had refrained earlier from giving it a 1.0 because the overlay becomes completely black. But I just added that :)

[thomaspark]

Yeah, there might be some confusion if someone goes to 100%, but there might be a need for all the way up to 99%.

Looks good, just one more thing. Now that opacity goes to 1.0, we probably also want the step value to change from 0.005 to 0.01.

[sukritchhabra]

Yup that makes sense. Just made that change :)