New fork: :highlighted_dates_check

[kikito]

I've created a fork that adds a new parameter to calendar_date_select,
called :highlighted_date_check.

It is similar to :valid_date_check, but simply "highlights" certain
dates without forbidding access to them.

For example, this highlights halloween (31st of October):
<%= calendar_date_select_tag "near_halloween_day",
"", :highlighted_date_check => "date.getDate() == 31 && date.getMonth
() == 10" %>
This highlights fridays:
<%= calendar_date_select_tag "casual_day",
"", :highlighted_date_check => "date.getDay() == 5" %>

I believe this should be useful for lots of people, so I've submitted
it for pulling.

I've created a test case on .tmp_cds_test.html, but I'm not sure I did
it correctly, please review.
I've modified the demo application and it seems to be working fine
with these new changes.

Others:
I also included an es.js locale.

The fork is available at http://github.com/kikito/calendar_date_select,
in case someone is interested.

Regards,

Enrique