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 this fix, 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.