Fixed a bug when subsection method is used twice with the same name
Fixed traceback when a section is not in schema (ref #6)
Added input_name for lexer

This solves a traceback error if t_error function is called since it uses
self._input_name for creating a Position object.
Passed input_name to parser

In Position object, a filename is stored to output if an exception
occurs (ParsingError, ValidationError). This value was never set since Dotconf
object never passed an input_name to the parser.