warn for geoms with showSelected=duration var but no key

[tdhock]

if we use smooth transitions a key should be specified to ensure that those transitions are meaningful. the compiler can and should warn the user if the key should be specified, but is not.

[coveralls]

Coverage decreased (-0.3%) to 0.907% when pulling 792e246 on warn-no-key into b0a708d on master.

[tdhock]

aes(href) is working for geom_point but not for geom_polygon. The a elements are rendered but they do not have an href attribute. this is probably an easy fix for the renderer.

[tdhock]

TODO add a test that captures #147

[coveralls]

Coverage decreased (-0.3%) to 0.907% when pulling f170343 on warn-no-key into b0a708d on master.

[coveralls]

Coverage decreased (-0.01%) to 1.198% when pulling 48a6376 on warn-no-key into b0a708d on master.

[coveralls]

Coverage decreased (-0.3%) to 0.899% when pulling 5437fb4 on warn-no-key into b0a708d on master.

[coveralls]

Coverage decreased (-0.01%) to 1.198% when pulling 3e9d779 on warn-no-key into b0a708d on master.

[coveralls]

Coverage decreased (-0.01%) to 1.198% when pulling 3e9d779 on warn-no-key into b0a708d on master.

[coveralls]

Coverage decreased (-0.3%) to 0.899% when pulling 42bdb47 on warn-no-key into b0a708d on master.