Changed the js to add a closeButton

Bound events to show/hide the close button when necessary.
Check when options are true.
Wrote a function to get closeButton when needed
index will pass in proper options.

Creates a sidebar with the right options.
L.control.sidebar('sidebar', {closeButton: true}
Changed the base style

_base.scss has a style for a close button and a helper class.
helper class is display none and helps hide the button
close button class has css that places and sizes the button
Ran it through gulp to modify all child css styles.