Revert "Remove Roo::Excel#formula, #forulas, #formula? outright, as they have always been placeholders"

This reverts commit 0599f386c15586926beb59c0cce10d0799fa9aff.
Instead of removing them entirely, raise NotImplementedError for Excel#forumla methods, as all the other classes implement them.
Rather than have separately implemented #formula? methods which rehash the #formula method, alias one as the other.

The only change is losing the boolean result, but that's the ruby way.
Remove unused requires.
Drop the TODO - all issues should be on github.
Readme whitespace.
Add examples for creating a CSV and TSV file. Fixes #14.
Refactor Roo::Base#open_from_uri to be DRY-er.
Rename Base#open_from_uri to #download_uri, and refactor CSV to rely on #foreach which can take an explicit encoding argument, unlike CSV.parse. Fixes #16.

Also adds an example to the readme.
Fix that #longest_sheet was in Base when it belongs in Excel (as it's written against the spreadsheet workbook)
Remove incorrect references to sheet().row_count from the readme. Fixes #45.
Expose Excel's Spreadsheet workbook as #workbook.
Pull #load_xml down into Roo::Base, and use it in Excel2003XML and OpenOffice.
Version bump to 1.12.0