Deligate equality to other if conversion to Unit fails.

[mrshannon]

This fixes issue #149 by returning NotImplemented if other is not or cannot be converted to a Unit. By doing this Unit allows other to make the decision or in the case that neither can determine equality Python will fallback on identity.

[coveralls]

Coverage decreased (-0.1%) to 88.489% when pulling 63bbb1a on mrshannon:fix_issue_149 into 812009c on SciTools:master.

[coveralls]

Coverage decreased (-0.1%) to 88.489% when pulling 63bbb1a on mrshannon:fix_issue_149 into 812009c on SciTools:master.

[coveralls]

Coverage increased (+2.8%) to 91.457% when pulling 95e99cc on mrshannon:fix_issue_149 into 812009c on SciTools:master.

[bjlittle]

@mrshannon Sorry for the very tardy response... I clearly missed this, but let's bank it now (finally) and include it in a patch release

[bjlittle]

@mrshannon LGTM 👍