Fix SPM support

[woxtu]

Hi,

The current release installed by SPM can't import headers as follows:

fatal error: 'PINRemoteImage/PINRemoteImage.h' file not found
#import <PINRemoteImage/PINRemoteImage.h>
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

And categories are not loaded:

error: no visible @interface for 'NSImageView' declares the selector 'pin_setImageFromURL:'
[imageView pin_setImageFromURL:[NSURL URLWithString:@"http://pinterest.com/kitten.jpg"]];
 ~~~~~~~~~ ^~~~~~~~~~~~~~~~~~~

This PR contains the following changes to fix that:

• Put header files back next to source files
• Create new public headers directory
• Add missing imports to the framework header

[grangej]

This does seem to fix the build but it seems to break the categories.

[woxtu]

@grangej Thank you for your pointing out. I could confirm that categories is not loaded, and maybe the current release has same issue. Anyway I'll fix that.

[grangej]

Awesome , any chance we can get this merged ??

[mthole]

Bump?

[mthole]

FYI: This may be taken care of by https://github.com/pinterest/PINRemoteImage/pull/586 already, which is merged to master but not in a release.