added additional ways of passing in images to Workbook._get_image_proper...

...ties

@jmcnamara
DONT MERGE ME (yet)

Hello!
I am in need of being able to pass an image as either a string or string buffer since I am working in a webapp and do not have access to files via file-path. I wrote some additional ways to open images in the Workbook._get_image_properties() method.

I need to run xlsxwriter's tests but can not figure out how to start them. Could you help me?

Thank you,
Michael Musslewhite