Some LESS work - operation type colors and fonts extracted to variables, divided spec.less to separated mixin files

Hi,
this change simplifies some UI look customization work:

 1 Colors - operation type color scheme (GET,POST...) can be easily changed by replacing the variable values in the palette.less file, thanks to LESS color functions one can optionally declare colors like:
@colorGetDark: #0f6ab4;
@colorGetMedium: lighten(@colorGetDark, 10%);
@colorGetLight: desaturate(lighten(@colorGetDark, 45%), 50%);

2. Fonts - as swagger-ui currently uses 2 font-family values, there are 2 global variables in fonts.less

3. Simple mixins - resource.less and operation.less, extracting that to separeted files seems to be natural, like splitting Coffee scripts to different files (views).

4. Parametrized mixin operationView.less (GET, POST...) shrinked some copy/paste code in this area.