change bsg_add_markup_class() to use arrays

bsg_add_markup_class() was using strings when adding class(es). The
combination of string usage and sanitize_html_class() prevented adding
more than one classes (spaces collapsed).

This change allows the initial $classes_to_add associative array to hold
the class as a string or array of strings.

It also changes the filter 'bsg-add-class' to now use an array instead
of a string.

Fixes #13
refactor handling string or array classes
Merge pull request #15 from salcode/add-mult-classes-to-markup

Add mult classes to markup
correct brace and paranthesis errors
remove sourcemap param on Grunt sass config

sourcemap now defaults to true to grunt-contrib-sass