Rename #add_child to #add_entry and alias as #<<.
Make EntryContainer Enumerable; add #last, #size methods.
Rename #contains_children_of_type -> #contains_entries_of_type
Rename and move parser_integration_spec under spec/unit/...
Split basic_parsing_spec into specific specs.

* Extracted:
    - basic_parsing_spec
    - orphan_parsing_spec
* TaskpaperUtils
    - Rename parse -> parse_file
    - Extract parse (takes an enum)
* Introduce SpecHelpers module for the long refactoring.
Move parser_spec -> parsing/parent_identification_spec
Clean up parent_identification_spec.
Extract high level specs out of raw_entry_spec.

* Introduce entry_elements_spec to hold text and trailing_tags specs.
* Move type identifications specs into basic_parsing_spec
* Remove raw_entry_specs
Minor cleanup.

* Removed extraneous describe block in basic_parsing_spec.
* Expanded orphan_parsing_spec into multiple examples.
Create spec/exploratory dir and remove indent_aware_spec.
Add spec to capture structure of a parsed document.

* Moved relavant specs from document_spec and entry_spec.
* Remove document_spec since rest of it was exploratory.
Extract behavior specs out of entry_spec and remove it.

* Created referencing_children_spec to hold specs for #[].
* Removed implementation specs for #text_with_trailing_tags.
* Moved/modified rest of specs into entry_elements_spec.
Remove exploratory specs in entry_container_spec.
Extract behavior specs out of entry_container_spec.

Introduce searching_by_tag_spec and document_tree_to_text_spec.
Clean up pending spec; remove obsolete ParsingHelpers.
Clean up straggling todo's from refactoring.