`app.jsx`: Added `handleGatheringsChange` & `handleRSVP`

`sideMenu.jsx`: Added "Gatherings"

`userListItem.jsx`: moved some methods into new `toggleStackListItem` mixin; Added generic `stackListItem` CSS class to allow broader CSS reuse

`remote.js`: Added `remote.parse.gatherings` methods (`getAll` & `rsvp`)

`heroes.jsx`: Moved some code into new `swipeStackSlider` mixin

`utils.js`: Added `momentFormatIfTime` method

`app.scss`: Changed `.userListItem` references to `.stackListItem`, to allow CSS reuse (from heroes to gatherings)

`src/script/screen/gatherings.jsx`, `src/script/component/gatheringListItem.jsx`: new screen, very similar to `heroes` screen

`src/script/mixin/swipeStackSlider.js`, `src/script/mixin/toggleStackListItem.js`: new mixins, abstracting out some of the code shared between `gatherings` & `heroes` (todo: abstract more!)