New `channel.views` to fetch daily view counts.

This commit introduces `has_many :views` for Channel.
Most of the code is similar to `has_many :earnings`.
The duplication will be removed in a separate commit.
Rename Channel#earning to Channel#earnings_on

The new name is coherent with views / views_on.

This commit also changes the *default* end date of Channel#earnings
from 1 day ago to 2 days ago, since YouTube does not provide earnings
for the previous day.
Merge branch 'add-views-to-metrics'