Add iOS targets to swiftz and swiftz_core

closes #59

The main goal of the pull request is the iOS targets swiftz_ios and swiftz_ios_core which are analogues to swiftz and swiftz_core.

But, in order to get swiftz to compile I had to add the swiftz_core.xcodeproj to the main swiftz  xcodeproj and add the dependencies in the appropriate build phases, so that change is also in this pull request.

also, as a convenience, added .DS_Store to the gitignore file.

I haven't done any iOS programming since about version 3.0 (90% of my coding has been Haskell for the last few years), so if this is unidiomatic, including the CPP guarded `import swiftz` vs `import swiftz_ios` statements, I'm happy to revise this.