Add ability to insert bitmap images from in-memory copies.

[rossigee]

This patch adds a new 'insert_bitmap_data' function on the Worksheet to complement the 'insert_bitmap' function. As the existing 'insert_bitmap' function depends on a filename as one if it's arguments, the new function allows programs that have a copy of the bitmap in memory to add it to the worksheet without first having to write it to file. (was PR #45)

[coveralls]

Coverage increased (+2.4%) to 57.945% when pulling 5469e8d on rossigee:insert-bitmap-from-memory-fix into 44f5acc on python-excel:master.

[coveralls]

Coverage increased (+2.4%) to 57.945% when pulling 9d24e82 on rossigee:insert-bitmap-from-memory-fix into 44f5acc on python-excel:master.

[coveralls]

Coverage increased (+2.6%) to 58.146% when pulling b5d0604 on rossigee:insert-bitmap-from-memory-fix into 44f5acc on python-excel:master.