Add well-border variable

Add a variable to allow the well border color to be specified.

The reason I'd like to see this included is that my companies styleguide
requires an explicit border color on well-like constructs. This variable
allows me to easily override the default to use that color.

less/variables.less

@@ -545,6 +545,7 @@
 // Wells
 // -------------------------
 @well-bg:                     #f5f5f5;
+@well-border:                 darken(@well-bg, 7%);
 
 
 // Badges

less/wells.less

@@ -9,7 +9,7 @@
   padding: 19px;
   margin-bottom: 20px;
   background-color: @well-bg;
-  border: 1px solid darken(@well-bg, 7%);
+  border: 1px solid @well-border;
   border-radius: @border-radius-base;
   .box-shadow(inset 0 1px 1px rgba(0,0,0,.05));
   blockquote {