Added ability to set default parameters; simplified gem

- Added ability to set default parameters via a `Minitext.set_defaults` method
- Added tests to check against defaults
- Removed `Minitext::Base` and moved all module level methods (including `gateway`) into `minitext.rb`
- Updated `MissingParameter` to refer to `StandardError` instead of `Exception`
- Added `guard` and `guard-minitest` to development group in rake task.  
Yeah, if we wanted to fix that we could provide both methods pretty easily.

```
def deliver!
  if valid?
    Minitext::Base.gateway.deliver(self)
    gateway.deliver(self)
  else
    raise_errors
  end
end

def deliver
  deliver!
rescue Minitext::MissingParameter
  false
end
```