Add other ButtonGroup examples + add styles for different ButtonGroup variations

[blazebarsamian]

I'm submitting a ...

• bug report
• feature request
• support request

What is the current behavior?
Currently we have some props that can added to ButtonGroups, however we don't have any styles written for those classes. We also only have one ButtonGroup example on the site.

What is the expected behavior?

• When we use any of the below ButtonGroup classes, there should be styles for each one.
• We should include a few more ButtonGroup examples on the examples page.

ButtonGroup classes that dont have any styles or need styles to be fixed:

• .rev-ButtonGroup--small

• .rev-ButtonGroup--large

• .rev-ButtonGroup--primary

• .rev-ButtonGroup--secondary

• .rev-ButtonGroup--small

• .rev-ButtonGroup--inverted

• .rev-ButtonGroup--success

• .rev-ButtonGroup--alert

• .rev-ButtonGroup--warning

• .rev-ButtonGroup--disabled

• .rev-ButtonGroup--expanded

• .rev-ButtonGroup--stackedForSmall

• we should probably add a stackedForMediumDown prop and styles too

For feature requests: why should the behavior change?
Because we should have props that actually work. Or else not have the prop at all.

[blazebarsamian]

• secondary button groups: check to see if box-shadow vs border gets rid of the gap between buttons