Added @gray-base variable to make @gray-{shade} reference, also allows @...

Also allows @gray-base to be modified with less#modifyVars (see [Using Less in the Browser](http://lesscss.org/usage/#using-less-in-the-browser)) and the vars to cascade.

The reason I did this is that as I mentioned it allows less to cascade the changes to the @gray variables if the @gray-base variable is changed.
It also makes sense as all the variables have the same value, so why not extract it into a single variable.

I have done something similar in my own [variables_custom.less](https://github.com/ilikeprograms/BootstrapThemeEditor/blob/master/src/variables-custom.less) file for my [BootstrapThemeEditor](https://github.com/ilikeprograms/BootstrapThemeEditor) project.

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