Support keywords [WIP]

Hi Bruno,

I'd like to use keywords via XML-RPC.
I think TestLink supports to add keywords via XML-RPC (but undocumented).

I wrote the code like below, I could create TestCase with the keywords "registered_keyword_A" and "registered_keyword_B" via API.

```java
        api = new TestLinkAPI(testlinkURL, devKey);
        TestCase testCase = api.createTestCase("TestCase", 15, project.getId(), "kondo", "summary", steps , "", importance,
                executionType, 0, null, false, ActionOnDuplicate.GENERATE_NEW,"registered_keyword_A, registered_keyword_B");
```

I added the parameter "keyword" to TestLinkAPI#createTestCase(). How do you think about it?