Refactor Parser::EntryPair (now Parser::ParentHound)

Extract small methods out of #determine_parent_if_equally_indented
and #identify_parent, each returning the parent if their conditions
are met and nil if not.  When chained together in the right order,
these small methods act as a poor-coder's rule-engine.
Add EntryContainer#[] for text based child referencing.
Remove ParentHound by making Parser stateful.

ParentHounds were throw-away objects created per line parsed and
moreover didn't really provide any great design simplification.  This
commit leaves parser_spec.rb (#parent_of) in a bit of a hanging state,
but refactoring that really needs entry referencing to work first.
Refactor parser_spec.

* Make direct use of Parser#parse instead of externally creating Entry
objects and hitting #identify_parent, which was too implementation
specific.
* Make use of new EntryContainer#[] to simplify referencing children.
Update readme to show child referencing
Add first-pass documentation; turn reek IrresponsibleModule back on.