#160 Refactor folders: Element, Container, and Exception
- Rename folder Section to Element
- Rename folder Exceptions to Exception
- Move Section, Header, Footer, Settings to Container folder
- Move Element\Footer\PreserveText to Element\PreserveText
Container abstract class
Extends `Container` abstract class into `Footnote`, `TextRun`, and `Cell` element class.
Adding more functionalities to containers:
- Table: Ability to add footnote in table cell
- Footnote: Ability to add image in footnote
- ListItem: Ability to add list item in header/footer
- CheckBox: Ability to add checkbox in header/footer
- Link: Ability to add link in header/footer
Add unit tests for refactoring results
Add element availability matrix and enable `addObject` in footnote and textrun
Remove HashTable and all related properties/methods. PHPWord doesn't (yet) need it.
Deprecate `createSection` in favor of `addSection`
Make one level Element folder structure (move Cell and Row up one level)
Merge branch 'develop' into #160-element-container
Remove some duplication on Word2007 writers
Create new Element abstract class
Fix some test error
DocBlock and use fixes
Remove unused ZipStreamWrapper.php and cleanup some scripts
Merge all Word2007 relationships writer
Refactor Media, addObject to header and footer
Move OOXML specific feature from Media to Word2007\Base
Add Media::reset(), Style::reset(), Footnote::reset(), and TOC::reset()
Fix failed test