Add start and end date to business support scheme model

Add an active? method to determine if the scheme is currently active or
not
Expose start and end dates to scheme CSV export
Add fields to scheme form for setting start / end date
Include MultiParamAttributes Mongoid module

Without this module, Rails can't parse the multi-field attributes and as
such the start and end dates wont be set from the form.
Refactor the active? method on scheme

Using integers means I don't have to rescue errors, which was a horrible
way of doing things. If dates are nil, they are set to 0 which means
they can be compared against without the risk of exceptions.
Refactor the active method.

The number processing way of doing it was pretty rough and not really
obvious. Create two private methods to determine if a scheme is started
or ended, and then active? method is just a combination of those.

has_started? and has_ended? could be made public easily enough but we
don't use them elsewhere so it makes no sense to right now.
Add explicit version to timecop gem
Fix timecop to specific version
Rewrite the export tests to be cleaner
Merge pull request #43 from alphagov/add-start-end-dates

Add start / end dates to schemes