Add Marker::onDidChange and ::onDidDestroy event subscription methods

Since we’re deprecating emissary, I manually implemented event pausing
to keep the event-kit library simpler.
Add TextBuffer::onDidChange and ::onDidUpdateMarkers
Add TextBuffer::onDidCreateMarker
:lipstick: Only use TextBuffer::onDidChange in text-buffer-spec
Add TextBuffer::onDidChangePath
Add TextBuffer::onWillSave and ::onDidSave
Add TextBuffer::onWillReload and ::onDidReload
Add TextBuffer::onDidDestroy
Add TextBuffer::onDidStopChanging
Add TextBuffer::onDidConflict
Add TextBuffer::onDidChangeModified
Document Marker event subscription methods
Document TextBuffer event subscription methods
Make Marker event subscription methods public

Nothing else has the extended/essential distinction currently and this
isn’t the purpose of this documentation pass.
Deprecate ::on in TextBuffer and Marker
Emit legacy Marker 'changed' event correctly
Place args after the description in documentation
Clarify ::onDidChange vs ::onDidStopChanging in the documentation
Merge pull request #27 from atom/ns-simplify-events

Add event subscription methods