Add headers to cloneOptions

Right now in order to pass predefined headers to client, I have to add them after creating client, like this:
```
var client = manta.createClient({
    sign: manta.privateKeySigner({...}),
    user: ...,
    url: ...
});
client.client.headers['X-Auth-Token'] = ...;
```
I'd prefer to have it in client constructor:
```
var client = manta.createClient({
    sign: manta.privateKeySigner({...}),
    user: ...,
    url: ...,
    headers: {
         'X-Auth-Token': ...
    }
});
```