Defines special keys for special keys

Some special keys have duplicate members, for example `-` is both `dash`
and `subtract`. This change makes it so that these special keys have
their own definitions as their english names. The mapping is:

subtract: 109
dash: 189
divide: 111
forward-slash: 191
decimal: 110
period: 190
Merge pull request #10 from matthewp/issue-10

Duplicate member in object definition