Added support for \n in text elements

[pbvs]

This is a pull request for fixing #1260 for discussion on this change. (resolves #1260)

In short there are two changes:

1. When writing CT_Text to XML \n is replaced by the cml element <w:cr />
2. When reading parsing xml the element <w:cr /> is replaced by \n I added a unit test to check that everything is replaced as expected.

Please let me know if i need to make additional changes.

Regards, Pieter