Reuse HTTP client

Previously, every request would instantiate a new HTTP client.

Use `http.DefaultClient`[1] instead, which is the default client that Go
uses for HTTP requests, e.g. `http.Get` uses it[2].

[1]: http://golang.org/src/pkg/net/http/client.go?#L74
[2]: http://golang.org/src/pkg/net/http/client.go?#L251
Change user agent to 'GOV.UK Crawler Worker'

Which better reflects the name of this project.