Add head_offset keyword argument to table methods.

The offset is used for stripping head rows at the top.

For example in folder_contents we have the selection header at the top,
which is merged into all column headers.
This change allows us to filter them.
Add as_text keyword argument to table methods.
Add folder_contents page object.
Merge pull request #19 from 4teamwork/jone-folder-contents-page-object

folder_contents page object