add timecop_link_to helper method

Users can add specific dates link to their views by passing a Date
object to timecop_link_to.
MainHelper included in client's application helper
rename helper method
spec for MainHelper#time_travel_to
add time_travel_to exception handling
spec for time_travel_to exception handling

simple spec to make sure an exception is caught when a non Date object
is passed to time_travel_to