Hera-py

Added
- Interactive debugging!
- Support for `PC_ret` and `FP_alt` named registers.
- Support for `include` directive.
- Runtime errors for use of `SWI` and `RTI` instructions now show line of code, and are only emitted once per program run.

Fixed
- `DSKIP` with a named constant argument works correctly.
- Symbols (labels, constants and data labels) cannot be redefined.

Added
- Static type-checking.
- `--verbose` and `--quiet` flags.
- Zero-prefixed octal numbers (with a warning), for backwards compatibility with HERA-C.
- Limited support for the `SWI` and `RTI` operations.
- Character literals can be used in HERA code.

Changed
- Order of flags in debugging output, to match HERA-C.
- Static data begins at memory address `0xc000`, per the HERA manual.

Fixed
- Label names can begin with a valid register (e.g., `R1_INIT`).
- Data statements no longer cause labels to resolve to the incorrect line number.

Removed
- `--no-dump-state` flag (use `--quiet` instead).

Added
- Data statements: `CONSTANT`, `DLABEL`, `INTEGER`, `LP_STRING`, and `DSKIP`.
- `SP` as an alias for `R15`.

Changed
- The `assembly` subcommand has been renamed to `preprocess`.

Added
- Support for Python 3.4 and 3.5.

Initial release with support for all HERA instructions and pseudo-instructions, excluding data statements.