Added webhook's authenticity check functionality

Added a Secure module with a method called "check_request_auth", that'll receive 3 parameters:

timestamp: the timestamp that was sent on the webhook request
token: the token that was sent on the webhook request
signature: the signature that was sent on the webhook request

Also, the Secure module gets a parameter called offset, that is used to check if the webhook request is new enough, preventing someone to copy a timestamp, token and signature and use them to make requests anytime.

```ruby
# will return true to up to 10 minutes old requests
@mailgun.secure(10).check_request_auth(timestamp, token, signature) 

# will return true to up to 5 minutes old requests (default)
@mailgun.secure.check_request_auth(timestamp, token, signature)
```

However, if you want to skip timestamp check, just pass false when you call the secure module:

```ruby
# will return true to any valid request, old or not
@mailgun.secure(false).check_request_auth(timestamp, token, signature)
```

Also, added a test helper called "generate_request_auth", that will simulate a timestamp, token and signature from Mailgun. To use it, just do:

```ruby
# require the helper
require './spec/helpers/mailgun_helper.rb'

# configure RSpec
RSpec.configure do |c|
  c.include MailgunHelper
end

# ...

# call it like this
timestamp, token, signature = generate_request_auth("some-api-key")
```

This helper is not global because it will probably be used only on "secure_spec.rb".