Modernize project, use ChemDoodle JSON, add 2D previews

Sorry there are a lot of changes bundled together here – I'll try and keep future pull requests more focused. 

- Updated project settings for Xcode 5.1 and OS X 10.9.
- Switched to Automatic Reference Counting (ARC). This means that a lot of retain/release memory management code is no longer needed, however casts between C types and Objective-C types must now be [bridged](http://clang.llvm.org/docs/AutomaticReferenceCounting.html#bridged-casts).
- Updated ChemDoodle to version 6.0.0. There is no longer a separate ChemDoodleWeb-libs.js file, everything comes under the main ChemDoodle namespace.
- Added some additional information to the README.
- Rearranged the preview logic: First an attempt is made to use Open Babel to produce ChemDoodle JSON. If this fails, the raw file content is read instead, using the file extension to choose the appropriate ChemDoodle file reader.
- If every atom in the molecule has zero z coordinates, a toggle button is displayed to switch between 2D and 3D representations.
- Added some more formats that Open Babel is able to read, and also some alternative file extensions and mime-types for the existing formats.