Initial versions of POSInputStreamLibrary and POSDataStructures

POSInputStreamLibrary contains NSInputStream implementation which uses ALAsset as its data source. The main features of POSBlobInputStream are the following:
- Synchronous and asynchronous working modes.
- Autorefresh after ALAsset invalidation.
- Smart caching of ALAsset while reading its data.
- Using NSStreamFileCurrentOffsetKey property for read offset specification.
More info at https://github.com/pavelosipov/POSInputStreamLibrary

POSDataStructures contains binary heap implementation. The following features differ him from CFBinaryHeapRef:
- Remove equal and identical objects from the heap.
- Objective-C interface instead of plain C.
More info at https://github.com/pavelosipov/POSDataStructures