Add rowId to fix an issue when importing file in Access

When importing a file generated by Seine into Access (only tested with 2010),
Access was complaining and the following error was displayed:
"Unexpected error from external database driver (1)"

=> Adding the row ID to the sheet1.xml file fixes the issue.
Empty cells should not create an entry in sharedStrings.xml
Fix file corruption when strings count is too large

The previous implementation only worked with less than 9999999 strings
in total. Having more strings than this number caused the sharedStrings.xml
file to become corrupted (because valid characters were overwritten).

=> Increased the default number to 9999999999999
=> Refactored the way the replacement of the default count is done
HHVM crashes when trying to create null directories
Merge pull request #10 from adrilo/master

Merge fixes from Adrien