Add &str function in the expression base
Prepare the SQL equivalent function for &str (i.e., the string cast
function).
Implement SPARQL STR function in the query parser handler.
Change the variable access policy to private.
Get method interface to get name and datatype.
Add an abstract method to override column datatype.
Add support for overriding datatype for xsd:gYearMonth, xsd:gMonthDat,
xsd:gYear, xsd:gMonth and xsd:Day.
Cast as mapping term to get the datatype.
Rename exception class.
Use AbstractXmlType to handle the proper lexical reprentation of values
for each XML datatype.
Log tidy.
Fix bug on assigning datatype label to Literal, it should be a URI
object.
Remove method leftover.
Code tidy.
Check the new SQL type first before changing the bTypeOverriden flag. If
the new type != old type then it is overriden.
Change the equivalences for Gregorian-* types (e.g., xsd:gYear,
xsd:gMonthDay) to SQL type. Since the XSD specification mentions that
Gregorian-* types only can be converted to xsd:String then the
equivalent type for SQL would be VARCHAR.

Source:
http://www.w3.org/TR/2002/WD-xquery-operators-20020816/#casting-from-primitive-to-primitive
Handle better parser exception for Extension expression.
Merge and solve conflict branch 'develop' into feature/sparql-str

Conflicts:
	src/com/obidea/semantika/mapping/sql/SqlMappingFactory.java
Add SqlCastString.
Change the constant name.