Refactored selectfield.scss into _variables and _selectfield.scss

[yogeshgadge]

Refactored selectfield.scss into _variables and _selectfield.scss.

Gulpfile changes: Copying scss files to dist Other minor gulpfile.js simplifications.

By refactoring we can let users of mdl-selectfield extend these variables and further customize in their project. For example someone may want to change color as in #34.

Other cases are where certain projects may not want to directly include the css into their index.html (e.g. Angular projects). With this change they can bake their scss using these scss.

[yogeshgadge]

@mebibou Please take a look.

[yogeshgadge]

@mebibou I realized you are publushing all the sources to npm - I guess then we don;t need this. Or you may plan to not publish the src to npm then I guess then this change will be useful.