Let collections load more/less data using skip & limit parameters

[Leonardo Ascione]

Currently collections are loading all objects at the same time, furthermore there is no way to load objects exceeding the limit imposed by Parse.com (100). Objects should be divided in chunks and NgParse.Collection should provide methods to check if other chunks are available for download, and, if necessary, fetch them.

• Add chunkSize constructor option to set how many objects should be retrieved concurrently
• Add a loadMore method to load next chunk.
• Add a hasMore method to check if all chunks have been fetched
• Internally, set skip & limit parameters on query to process chunks