Implemented builder for adapted HtmlUnitDriver

fixes #40  - Implemented a builder for HtmlUnitDriver subclass (internal) that uses MockMvcWebConnection. Implemented builder method to allow direct definition of MockMvcWebConnection to use.

The context path can be defined with:

```
String contextPath = .....
MockMvc mockMvc = ....
MockMvcWebConnection con = new MockMvcWebConnection(mockMvc, contextPath);

HtmlUnitDriver driver = MockMvcHtmlUnitDriverBuilder.via(con).build();
```