Improved Word2007 Reader to read images when pictures is between <wp:anchor>

[begnini]

Description

Word2007 Reader didn't read images where pictures was between wp:anchor, accepction only with wp:inline.

The OpenOffice XML docs says:

The XML for placement of the picture (e.g., whether it is inline [wp:inline] or anchored [wp:anchor]) is within the wordprocessing drawingML namespace: xmlns:wp="http://schemas.openxmlformats.org/drawingml/2006/wordprocessingDrawing".

Improved tests to be capable of read images from docs generated by AbstractTestReader::getDocumentFromString

Checklist:

• I have run composer run-script check --timeout=0 and no errors were reported
• The new code is covered by unit tests (check build/coverage for coverage report)
• I have updated the documentation to describe the changes

[begnini]

This solves the same problem pointed by PR #1794. The difference between the PRs is the tests I added.

[coveralls]

Coverage increased (+0.009%) to 94.642% when pulling 6e463bb on begnini:better_image_reader into 733f845 on PHPOffice:develop.