Ensure <cascade/> elements preceed <join-columns/>

When converting annotations based metadata to XML, the position of the <cascade/> elements resulted in XSD validation errors caused by the definitions of the association ComplexTypes:
- https://github.com/doctrine/doctrine2/blob/master/doctrine-mapping.xsd#L463
- https://github.com/doctrine/doctrine2/blob/master/doctrine-mapping.xsd#L482
- https://github.com/doctrine/doctrine2/blob/master/doctrine-mapping.xsd#L495
- https://github.com/doctrine/doctrine2/blob/master/doctrine-mapping.xsd#L517

Since changing the XSD might result in validation problems in existing mappings, changing the XmlExporter seems a better approach.

Note that the '<cache/>' element is not yet generated by the XmlExporter, but according to the XML Schema, it should precede the '<cascade/>' element.
Respected 'inheritanceType' at Entity level

It was (incorrectly) fetched from Table-level.
http://docs.doctrine-project.org/projects/doctrine-orm/en/latest/reference/annotations-reference.html#annref-inheritancetype
Exclude exporting inheritanceType for INHERITANCE_TYPE_NONE
Merge pull request #897 from holtkamp/patch-3

Respected 'inheritanceType' at Entity level
Merge pull request #896 from holtkamp/patch-2

Ensure <cascade/> elements preceed <join-columns/>
fix connection mock fetchColumn signature
Merge pull request #900 from deeky666/fix-connection-mock

Fix connection mock fetchColumn signature
exclude unsupported HHVM + PostgreSQL and HHVM + Mysqli from travis build matrix for now
Merge pull request #901 from deeky666/disable-travis-hhvm-unsupported-drivers

Exclude HHVM + PostgreSQL and HHVM + Mysqli from travis build matrix
removed erroneous tip

I'm working through the tutorial myself and got to this point.

Don’t forget to add a require_once call for this class to the bootstrap.php

This advice is wrong. The class is located in the src directory and so is autoloaded when required. No require statement is necessary.
Add MyISAM limitation
Merge pull request #906 from KevinBrogan/master

removed erroneous tip
add missing use statement for SQL Server testsuite to work
Merge pull request #909 from deeky666/fix-sqlserver-database-driver-test

Fix DatabaseDriverTest on SQL Server
fix foreach coding style
Merge pull request #911 from goatherd/fix-foreach-as-style

fix foreach coding style
Added exception class names
added license badge
Merge pull request #914 from liuggio/patch-2

added license badge
Merge pull request #913 from WouterJ/patch-1

Added exception class names