add get-all method so we can use it to migrate to different retention structures
fix ceil; e.g. (ceil 10 10) should not round up to 20
do not permit negative values for from or until
change get-all to return a range for every archive resolution in the database
add get-all-points which stitches points together from different resolutions
Miscellaneous cleanup
Another approach to get-all-points
Trade a little performance for code size
Use clojure.core/list to emphasize listiness.

The vector literal made it easy to not notice that the levels of
nesting were different.
add values to get-all and get-all-points tests
add migrate! for moving data to a new retention structure
fix below-density tests
Add a clear function that removes all values from a database.
Fix test:

Verify that files are not-equal before clearing, as well as equal
after clearing.
Can actuall compare more than just :values
Merge branch 'archive-migration' (early part)
Version 0.1.5