Add PHAsset support in place of ALAsset

[wibs]

ALAsset is deprecated as of 8.0 in favor of PHAsset, and there are a couple UIImage extensions that deal with ALAsset and ALAssetOrientation. So, if we want to continue supporting 7.0, we should keep ALAsset in, but we should match those features with their PHAsset equivalents.

[nhgrif]

We do not want to support iOS 7. Minimum version has been bumped to iOS 8.

[wibs]

Since we aren't supporting iOS 7, we'll be dropping ALAsset support and replacing it with equivalent functionality for PHAsset

[wibs]

I misread the Apple docs for this, ALAsset was not deprecated until iOS 9. However, the Photos framework was introduced in iOS 8 and is what Apple favors over ALAsset. After looking more into what our UIImage extensions do regarding ALAsset, it seems like the Photos framework handles most of what we were trying to accomplish. I'd suggest we just deprecate tc_imageFromAsset and tc_imageOrientationFromAssetOrientation, since both of those expect ALAsset properties and those are not compatible with PHAsset objects.

[lgauthier]

@wibs @nhgrif Are we ready to just remove these methods now and bump up to v3.0.0? I'm pretty tired of seeing the warning in all my projects =(