Add responsiveness for footer. Update gulpjs, package.json and bootstrap sass

static/assets/bootstrap/sass/_mixins.sass

@@ -0,0 +1,41 @@
+// Toggles
+//
+// Used in conjunction with global variables to enable certain theme features.
+
+// Utilities
+@import mixins/breakpoints
+@import mixins/hover
+@import mixins/image
+@import mixins/badge
+@import mixins/resize
+@import mixins/screen-reader
+@import mixins/size
+@import mixins/reset-text
+@import mixins/text-emphasis
+@import mixins/text-hide
+@import mixins/text-truncate
+@import mixins/visibility
+
+// // Components
+@import mixins/alert
+@import mixins/buttons
+@import mixins/caret
+@import mixins/pagination
+@import mixins/lists
+@import mixins/list-group
+@import mixins/nav-divider
+@import mixins/forms
+@import mixins/table-row
+
+// // Skins
+@import mixins/background-variant
+@import mixins/border-radius
+@import mixins/box-shadow
+@import mixins/gradients
+@import mixins/transition
+
+// // Layout
+@import mixins/clearfix
+@import mixins/grid-framework
+@import mixins/grid
+@import mixins/float