Money.default_currency=() accepts a lambda.

Hi,

This pull request modifies Money to accept a lambda when setting a default_currency. 

This is useful for multithreaded environments, like a Ruby on Rails application running on Puma. I am using it like this:

````ruby
MoneyRails.configure do |config|

  # To set the default currency
  config.default_currency = lambda { RequestStore.store[:default_currency] || :eur }
end
````

````ruby
class ApplicationController < ActionController::Base

  before_filter :set_currency

  def set_currency
    RequestStore.store[:default_currency] = current_clinic.currency
  end
end
```

There are some minor changes to do on money-rails for this to work. I will make PR too in that repo if this gets merged in.
Feel free to submit a PR.