Replace embedded _links with Link(-Template) header

Fix #205. As noted in the conversation on #205 itself, this will take some updating by existing clients to read and process the Link and Link-Template sections from the header. The Link portion should be a very straightforward update as it behaves exactly as `_links` did but some more care will need to be taken when processing the Link-Template headers as they provide templates of URLs that will need to be adapted to individual objects based on id or other field replacements.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
Since this pattern is repeated often below, please create a function that wraps `reverse` to handle this case. Something like:

```python
def reverse_tmpl(name, params):
    kwargs = {}
    variables = {}
    
    for key, type in params.items():
        # Initialize the "zero" value of the type
        value = type()
        variables[str(value)] = '{{{0}}}'.format(key)
        kwargs[key] = value
            
    url = reverse(name, kwargs=kwargs)
    for val, var in variables.items():
        url = url.replace(val, var)

    return url
```

Usage would be:

```python
reverse_tmpl('serrano:category', {'id': int})
```