Issue-818. Issue.Smart#exists() implemented.

@carlosmiranda It looks strange I understand. But it's only way like Qulice works. My first version was:
```
.assertStatus(
    Matchers.isOneOf(
        HttpURLConnection.HTTP_OK,
        HttpURLConnection.HTTP_NOT_FOUND
    )
```
but Qulice wants:
```
.assertStatus(
    Matchers.isOneOf(
        HttpURLConnection.HTTP_OK,
        HttpURLConnection.HTTP_NOT_FOUND
)
```