added ability to manually specify file type on .open
fixes parsing csv with headers option
Recognize 1904 and 1900 as potential base dates in xlsx

Modern excels use 1900 as the base date when storing dates as numbers,
but Excel for Mac < 2011 uses 1904! Roo already supported the
different offsets in .xls files but didn't support the offsets in
.xlsx format. This change adds support for recognizing the base date
format in .xlsx files.

1904 vs 1900 is configured in the workbookPr node of workbook.xml.

    http://msdn.microsoft.com/en-us/library/ff530155(v=office.12).aspx

There will be no difference in behavior for any spreadsheet generated
by any version of Windows excel or Excel for Mac 2011.
Be more careful about checking for boolean true

Excel doesn't add a date1904 attribute when it's false, but some other
libraries due (like the axlsx writing gem) and it's in the spec.
Merge pull request #38 from scpike/excel-for-mac-date-system

Recognize 1904 and 1900 as potential base dates in xlsx
attempting update to change google_spreadsheet to google_drive
Add note about spreedsheets license
Merge pull request #43 from sleeptillseven/master

Add note on spreedsheet license
Correct some typos and slightly simplify the "Spreadsheet's License" section
Further refine the spreadheet's license section and make it more Roo-centric.
added the ability to pull links from excel cells
added ability for links in xlsx files
performance fix for huge xlsx files with links.

Removed the path attribute lookup and cached the rels before the hyperlink loop
checking for nil rels. oops.
Fix typo in README.markdown
Moved CHARGUESS constant inside Roo::Excel class
Merge pull request #54 from ikido/master

Moved CHARGUESS constant inside Roo::Excel class
Merge pull request #52 from kbroman/master

Fix typo in README.markdown
Merge pull request #37 from vyrak/parse_with_headers_option

fixes parsing csv with headers option
Merge pull request #40 from Kagetsuki/master

Changed Google Spreadsheet to Google Drive