Drop pry from the dependencies.
Drop deprecated behavior in prep for roo 2
Split LibraOffice into its own file.
Move open office file to match its class name.
Prefer tr over simple gsub.
Pull out Roo::Excel, Excel2003XMl, and Google, to be provided in separate libraries, roo-xls and roo-google.
Avoid using single-character variable names.
Switch from Spreadsheet::Link to Roo::Link, thus removing spreadsheet dependency.

Also change the behavior of Excelx#celltype to output :link in the proper cases.
Update the license copyright date.
Prefer 'add_dependency'
Use method-level rescue.
Fail expressively when #row_with fails to find the headers. #129
Back-fill changelog.