Following Apple's guidelines for iOS Static Libraries

Cocoa Lumberjack Team,

This pull request may not interest you given the continued effort put into maintaining the CocoaPods spec, but I'm optimistic that there are others like me that prefer to use bare git submodules and a static library.

I've reworked the iOS static library project to follow Apple's recommendations in [this tech note][1]. If the Apple link changes, then you should be able to find it by searching the phrase "Using Static Libraries on iOS." This puts exported headers in an expected place (however much water that may hold), and does not require changes to a target app's `Search Paths` build settings.

I've found these modifications to the Lumberjack static lib helpful for my projects. Maybe others will, too.

I welcome discussion about why you think this is a beneficial or not-so-great pull request; this is my first contribution to the community and I'm hoping to learn from the experience. I've enjoyed using CocoaLumberjack, and I'm hoping this might be a small way to show my support.

[1]: https://developer.apple.com/library/ios/technotes/iOSStaticLibraries/iOSStaticLibraries.pdf