fix(position): improve placing around corners

[ksheedlo]

This change improves corner positioning to make it more internally consistent and useful.

The current implementation of corner positioning aligns elements in a way that is less useful than it could be. For instance, passing left as the second position argument aligns the left edge of the target element with the left edge of the host, while passing right aligns the left edge of the target with the right edge of the host. This behavior makes it difficult to trigger popovers on a right-aligned control. With this PR, passing right as the second position argument will cause the right edge of the target to be aligned with the right edge of the host control; and likewise for top and bottom. Here is the plunker with the updated functionality.

This change is potentially breaking, but I feel it's worth it in terms of improving the usability of positioned elements in ui-bootstrap.

[wesleycho]

Thanks for this PR - I will try to review this this week, but it may be difficult since I am heading to a week long vacation in a couple of days.

This generally looks good, but I just want to think about the API signature some and whether we could improve it.

[wesleycho]

Thinking about this some more, can you demonstrate this in a Plunker? This may potentially cause issues if the user wants it aligned to the right but the host is not aligned to the right.

[ksheedlo]

@wesleycho Plunker

[icfantv]

Closing in favor of #4762.