add importer for XLS and XLSX

[jorol]

• add importer for XLS and XLSX files
• add exporter for XLSX
• add test for importer and exporter
• add XLS and XLSX test files
• add prerequisites to Build.PL
• modify pod
• edit README
• switch to Dist::Milla

[nics]

Hi Jorol, sorry this took so long. Sent to cpan as 0.04.