Update Xml and String docs.
Update more docs

* Fix code samples
* Add headings to make TOC scanning simpler.
Update a few more chapters.
Merge branch '3.0' into 3.0-proof-pt4

Conflicts:
	en/installation/url-rewriting.rst
Fix mistakes and add stackTrace() function.
Fixed var name casing.
Merge pull request #1551 from cakephp/3.0-proof-pt4

3.0 proof pt4