Accept use of relative date literals as time values

Fixes #26 (closed)

Adds relative date literals to the language.

Now, instead of doing:

• created > startOfDay("-7")

You can do:

• created > -7d

It also allows to work with other time-spans than just days.

The following denominators are supported:

• d for days
• w for weeks
• y for years (always 365 days)

This provides the first example of something resembling "bare words" in the parser. This could provide a basis for the implementation of Implement ISO date literal (yyyy-mm-dd) (#27 - closed) also.

Why not months?

Months are tricky both in implementation and meaning. What does it mean to subtract 1 month from the 31st May? 30th April? 1st May?

For %0.0.5 I think it's fine to omit months and anything at a lower resolution than one day.