Add assembler project structure, setup cucumber.

Write first test, stub out system testing structure, add development
dependencies.
Implement test helpers, write first system test.
Include load path in bin.
First cut of line parser.
Implement integer encoder.
Build jump encoder.
Build out comp code encoder.
Clarify test for source line.
Build basic destination encoder.
Fix bugs with parsing, make main encoder.
Wire together symbol-less assembler.
Build symbol table.
Finish implementation of symbol lookup.
Massive refactor of classes.
Fix code quality and add reversals.
Add invalid computation error.
Remove encoder concept, add more error handling.
Refactor computation parsing.
Refactor out parsing logic to instructions.
Refactor and adjust parsing.