POP->pop to reflect filesystem naming

Fixes #78.

Using POP is unwise as git won't necessarily update people on case-insensitive filesystems, I think. In addition, the pod is named 'pop', rather than 'POP', and it's a good idea to `#import <podnamehere/SomeHeader.h>` for consistency.

Having non-case-sensitive folks either re-pull or potentially deal with case problems if they move their repo to a case-sensitive filesystem doesn't seem like a good idea; going with "POP" on the filesystem would probably be a significantly negative developer experience given that 'pop' is more consistent and what's already on disk.

@kimon this really isn't a CocoaPods issue, as the podspec correctly identifies the filesystem paths as pop/whatever. It does go a little wild on specifying exported headers explicitly instead of using the folder hierarchy for conciseness, but that's not a big deal.

I've modified all references to the pop folder. I checked as such:
```
% git grep -h "POP" | grep import | sort -u
% git grep -h "POP/" | sort -u
% git grep -h "pop/" | sort -u
```