Liblet

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- Adding a quickstart generated via Jupyter.

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- Working on documentation.
- Now matches can be restricted to prod or pos, used to implement
left/rightmost steps.
- Made Derivation immutable.
- Repurposed display.productions_table in the more general display.iter2table.

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- Now left/right-hand side are separated with HAIR_SPACE.
- ProductionGraph now works by level (every step goes down one level).
- Checking that travis gets a failure.
- Added Travis integration.
- Added code to test if ANTLR4_JAR is defined and points to an existing file.