Improvements on parser

Implemented syntax error messages, added debug names to each of the
parser expressions, tested at least one case for each of the currently
implemented features. Syntax error message is limited due to loss of
info on some error-tolerant pyparsing expressions. To get near of the
error spot, it would be necessary to prevent backtracking. See:
http://stackoverflow.com/questions/15897094/improving-error-messages-with-pyparsing