Add max width to .select, .select select (#719)

* Set .select, .select select max-width to 100%

* Comply with styleguide

sass/elements/form.sass

@@ -98,6 +98,7 @@ $input-radius:              $radius !default
 .select
   display: inline-block
   height: 2.25em
+  max-width: 100%
   position: relative
   vertical-align: top
   &:after
@@ -111,6 +112,7 @@ $input-radius:              $radius !default
     cursor: pointer
     display: block
     font-size: 1em
+    max-width: 100%
     outline: none
     padding-right: 2.5em
     &:hover