fix(popoverTriggerDirective): use popover-id if provided

Previously, the popover trigger would toggle the popover that contained the
trigger, even if a popover was provided. This prevented using a popover trigger
to open a nested popover. This change updates the popover trigger to always use
the popover-id attribute if provided.
feat(popoverTriggerDirective): remove custom popover trigger events

This option is just bloat and a distraction right now. In the near term, this
can easily be implemented by consumers of the directive. In the long term, we
can easily add this option back if there is a clear need.
feat(popoverTriggerDirective): simplify trigger interface

Only allow the re-popover-trigger directive as an attribute. Allowing many
different usages of rs-popover-trigger caused unnecessary bloat to the
interface.
feat(popoverTrigger): reposition popover when shown
feat(tether): implement basic positioning algorithm
Merge pull request #5 from rackerlabs/popover-positioning

Popover Positioning