New constructor and some more methods.

Constructor now uses an associative array of options, including now
optional filename.
Add getFileName() method to access the given or
temporary filename.
Add toString() method to return the workbook binary
data as a string.
Modified to act as a drop-in compatible replacement
for PEAR's Spreadsheet_Excel_Writer, as far as I need:
* Add
setCustomColor() method (as alias of set_custom_color).
* Add send()
method.