Added params option

When I changed the month the existing get parameter get lost, so
I added the params option to the calender helper.
Now I can keep or change them just as I like.

e.g.:
<%= calendar @events, {params: {type: "birthday"}} do |event| %>
  <div><%= link_to event.title, event %></div>
<% end %>
Merge pull request #30 from Jupiterrr/params-option

Added params option