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.  
Why don't we just dump the default gateway into the default params when you hit the accessor. Something like:

```
def self.defaults
  {gateway: @@gateway}.merge(@@defaults)
end

def self.defaults=(defaults)
  @@defaults = defaults
end

def self.text(params)
  Message.new(defaults.merge(params))
end
```