Add a `-libc++` compiler flag to the podspec
Lots of improvements to the library

- Added encoder/decoder blocks for turning objects into NSData (fallback to NSKeyedArchiver if absent)
- Made keys NSData by default, but NSString possible for setting and getting objects (UTF8 encoding is assumed)
- Made as much of the API mimic NSMutableDictionary
- Added a `safe` property to control leveldb's `fsync` behavior
- Added a `-initWithPath:andCacheSize` initializer method to take advantage of leveldb's builtin LRU cache