update schema XSD definition to allow for specifying a column's singular form
Use Column->getSingularName() where applicable
Introduce Column->getPhpSingularName() and use it where appropriate. The
phpSingularName property of Column is initialised using the new
'phpSingularName' attribute for columns in the schema, or (if not specified in
schema) generated based on the column's phpName.

Minor update of comments to remove grocer's apostrophe (possessive form is
"its" rather than "it's")
don't tell lies about the origin of the auto-generated value
Expand ColumnTest to include testcases for new singular form phpName feature
Merge pull request #638 from sjvs/features/override-column-singular-form

Introduce attribute to override singular form of a column's phpName