Allow specifying time zone for dates when importing

[martinstuder]

"I just discovered your package XLConnect and find it very useful. Before, I used read.xls() from package gdata, but the columns containing dates were imported in unpredictable manner and I had to do some work to convert to valid dates.

Your package does this automatically, which is wonderful. I just wish we could tive a time zone when calling your functions to import Excel sheets. I keep my computers doing data acquisition to standard time in summer (i.e. I do not switch to advanced summer time) for various reasons, and they import in advanced time with readWorksheetFromFile(). I sometimes receive data from colleagues in diverse locations and I cannot specify the appropriate time zone for those occasions either.

I manage after importing, but then it is hardly more straightforward than using gdata."