proposal: add support for github.com/bradfitz/gomemcache

[dd-caleb]

The github.com/bradfitz/gomemcache package seems to be the most popular memcache client. We should add tracing support.

Looking at the library, it doesn't currently support Context, so I submitted an issue: bradfitz/gomemcache#84.

Regardless we can create a new Client that wraps the memcache.Client, implements all the same methods, and also supports a WithContext method:

func (c *Client) WithContext(ctx context.Context) *Client

Then subsequent calls on the new Client would use that context as the parent for spans.