Make the options setter append the options instead of replacing them

Might fix issue 26 and issue 39

Certain Amazon Product APIs such as es, it or cn require that the options include
```ruby
:service => "AWSECommerceService"
```
in the query. These options are set by default on ```Amazon::Ec2.options```, but configuring the gem like
```ruby
Amazon::Ecs.options = {
  :associate_tag => '[your associate tag]',
  :AWS_access_key_id => '[your developer token]',       
  :AWS_secret_key => '[your secret access key]'
}
```
overrides them. With these fixes, the options now are merged with the ones previously set. Another maybe cleaner way to go would be to separate the ```defaults``` into a separate variable and manually merge them before each request.