Improve container

Created by: matplane

I suggest this improvement on the .container mixin. I think it's a nice way to use containers with multiple grid system.

Suggested new mixins with @gutter param.

My pull request is ready if you are ok with that.

.container-fixed(@gutter: @grid-gutter-width) {
  margin-right: auto;
  margin-left: auto;
  padding-left:  (@gutter / 2);
  padding-right: (@gutter / 2);
  &:extend(.clearfix all);
}

.container(@gutter: @grid-gutter-width) {
  .container-fixed(@gutter);

  @media (min-width: @screen-sm-min) {
    width: @container-sm;
  }
  @media (min-width: @screen-md-min) {
    width: @container-md;
  }
  @media (min-width: @screen-lg-min) {
    width: @container-lg;
  }
}