Add an easy way of adding Authorization header

Resolves #84
Closes #85

Thanks to @stewart for initial work on this feature.

### Example of usage:

``` ruby
HTTP.auth("true-lah-lah")
# Authorization: true-lah-lah

HTTP.auth(:basic, :user => 'user', :pass => 'pass')
# Authorization: Basic dXNlcjpwYXNz

HTTP.auth(:bearer, :token => 'foobar')
# Authorization: Bearer foobar

HTTP.auth(:bearer, :token => 'foobar', :encode => true)
# Authorization: Bearer Zm9vYmFy
```