fix: iss#88 raises on image w/uppercase ext

Calling add_picture() with an image file path having an uppercase
filename extension caused ValueError to be raised even though image
file format is supported.
fix: iss#89 raises on add_slide() w/duplicate id

pptx.oxml.presentation.CT_SlideIdList._next_id was not sophisticated
enough to detect duplicate ids when adding a slide to an existing
presentation.