Adds support for Excelx (xlsx) Exporter

[athityakumar]

• Fixes a silly yet major bug with linkage between daru & daru-io calls (for ALL write exporter methods, which yet again draws attention to handle #55 ASAP)
• Adds support to Excelx Exporter (formatting support not added yet) with documentation and tests. Current arguments include sheet and boolean values for index, header and data (show them or not). And of course, filename for write method. Fix #63.
• All the write methods return true incase of no mishaps when the particular exporter is called.

[athityakumar]

Oops, I didn't notice that this Pull Request had already been reviewed. I'll soon push the changes for the next review. Meanwhile, I've also opened #71 for keeping track of, in a different PR.