Support for streamed parsing of very large excelx sheets

Changes should address the following issues:
read_cells was being called immediately (via to_s call from xml parser) when opening any excelx document, which impacted initial load time, not sure what the preference for fixing this is. For now, to_s will not evaluate on an unloaded sheet.

Added each_row interface which yields each row (as array of Roo::Excelx::Cell objects) to a given block. It is designed to run without first calling read_cells. Should improve perf in cases where you only want to iterate over rows and don't need all the other niceties of the interface. However it still requires the entire xml document (this is the current behavior) to be loaded into memory. 

Finally, added additional initialization option, :minimal_load. Which skips the loading of ALL sheets, comments, and styles. Allowing large files to load much faster and with minimal memory footprint. Paired with minimal_load is each_row_streaming which will open only the default sheet and use a streaming approach to reading rows, which are yielded to a given block. This DOES NOT load the whole sheet xml document into memory, meaning it should work with even the largest of documents**. Currently streaming does not respect styles, or comments, but supporting that should be simple enough. 

**This still loads the entire shared string xml document into memory, which could get pretty big for huge files.