Add multiply, invert and screen methods

[benjamminf]

Added the blend modes multiply and screen as well as a method to invert the color. Test cases provided.

Usage:

Spectra(...).multiply(...);
Spectra(...).screen(...);
Spectra(...).invert();

[benjamminf]

Have a fair few more blending modes to go but thought I'd go ahead and commit these.

[coveralls]

Coverage remained the same when pulling 4a4081a9f2ce6fe3bc2c3902207c09645c8fc11a on benjamminf:master into 6c3fa2c556bdf3e4d28b4becab67f9b401827e5d on aakpat6:master.

[avp]

I should be able to merge this when you fix those two comments.