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