UIActivityIndicator

[xamelon]

Added UIActivityIndicator when image is downloading

[evgenyneu]

Hi @xamelon, good idea. Here are things that need to be done before it is merged:

• Create activity indicator when image starts downloading, and not on init, since download does not necessarily needs to start after the image view is created.

• Handle case when one moa image view displays multiple images in succession: one after another. In each case, indicator should be created on download and removed when it finishes/fails.

• Only create activity indicator object when it is needed. Don't create it by default, since most current users don't need it, or don't expect it to be there. This can be done by adding a setting moa.showActivityIndicator = true, which is false by default.

• Position activity indicator using auto layout constraints instead of frame coordinates. This will ensure the indicator remains in place when image view changes size (on phone orientation change, for example).

• Add unit test.

• Add documentation to readme.