Collapsable Mixin fix

Fixes 2 issues with collapsable panels in current master:

- https://github.com/react-bootstrap/react-bootstrap/issues/265
  - Panel with defaultExpanded={false} doesn't render correctly when opened
  - BROKEN = http://screencast.com/t/EAizMEI8JSh
  - FIXED = http://screencast.com/t/mSDw54i6

- https://github.com/react-bootstrap/react-bootstrap/issues/266
  - Panel Group animation has become irregular / unpredictable
  - BROKEN = http://screencast.com/t/QnFadPU2hU8o
  - FIXED = http://screencast.com/t/NhNwQsu8ePrm

- also still works with content that changes size (the original problem that the master changes was originally fixing)
  - http://screencast.com/t/wBQAJZXF
- also now allows for rapid clicking of panel headers to interrupt / redirect the animated transition in flight
  - http://screencast.com/t/F0YQDxW8G

@stevoland 
@pieterv