feat(tooltip): add *-append-to-body attribute

The tooltip and popover directives (through the $tooltip service) now
support using an attribute in addition to the provider to set a
particular tooltip or popover to use $body as the container for the
popup element rather than the directive element's parent.

Closes #395.
feat(tooltip): add custom trigger support

The $tooltipProvider now allows extending the default set of open and
close trigger mappings.

Closes #382.
docs(tooltip): add $tooltipProvider docs

Until now, the $tooltipProvider has been an undocumented feature, but as
there are now several options requested by the community at large, it is
finally included in the documentation.