Rename "pry_history_spec.rb" to "history_spec.rb"
History: increment `@original_lines`

Fix broken `Pry::History#session_line_count`. Add a test.
Command::Hist: change the behaviour

Fix issue #205 (history option to show just history for current session)

Add `--all` switch. `hist` is `hist --all` now. `hist` without any
parameters shows the history of the current session.