* introduced YamlSequentialParser object that stores
all known tag URIs
* Added procs for registering and querying tag URIs
* Return non-specific tags instead of nil if no tags
exist in source
* Lexer: column now always points to start of token
* Lexer: added ability to compile with -d:yamlDebug for debug output
* Parser: Correctly handle nested sequences and maps
* Tests: Added parser tests for nested objects