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 4a4081a on benjamminf:master into 6c3fa2c on aakpat6:master.

[avp]

This seems to be a duplicate of negate? Consider aliasing one to the other - I don't really care which.

[avp]

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