Replace house identifiers with constants.

This pull request replaces instances of the house identifier numbers with defined constants, which makes the code and its intent more readable.

* Closes #192.

## Houses

`0`: Special case for members of the Royal Family (currently Queen Elizabeth II)
`1`: House of Commons (Westminister)
`2`: House of Lords (Westminister)
`3`: Northern Ireland Assembly
`4`: Scottish Parliament
`5`: Assembly for Wales (Not currently in use, but pre-emptively reserved)