Minify css files on build using cssmin

In order to push ngDialog to cdnjs we need to minify CSS as well as JS(See requirements at https://github.com/cdnjs/cdnjs#pull-request-pre-flight-checks )

I've added minificaion of css using cssmin.
Introduced three new ".min.css" files in src directory.
Cssmin added to build process(for tasks "build" and "css").
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Size comparison of minified/non minified css:
File css/ngDialog.min.css : 1.6 kB → 1.33 kB
File css/ngDialog-theme-default.min.css : 4.47 kB → 3.85 kB
File css/ngDialog-theme-plain.min.css : 3.45 kB → 3.12 kB