Merge pull request react-bootstrap#1069 from AlexKVal/bsSize

Add 'xs, sm, md, lg' values for 'bsSize'

Author: AlexKVal

src/BootstrapMixin.js

@@ -16,7 +16,7 @@ const BootstrapMixin = {
     bsStyle: React.PropTypes.oneOf(styleMaps.STYLES),
     /**
      * Size variants
-     * @type {("xsmall"|"small"|"medium"|"large")}
+     * @type {("xsmall"|"small"|"medium"|"large"|"xs"|"sm"|"md"|"lg")}
      */
     bsSize: CustomPropTypes.keyOf(styleMaps.SIZES)
   },

src/styleMaps.js

@@ -40,7 +40,11 @@ const styleMaps = {
     'large': 'lg',
     'medium': 'md',
     'small': 'sm',
-    'xsmall': 'xs'
+    'xsmall': 'xs',
+    'lg': 'lg',
+    'md': 'md',
+    'sm': 'sm',
+    'xs': 'xs'
   },
   GLYPHS: [
     'asterisk',