Add method fetch for Hyperclient::Resource

[alabeduarte]

Hi all,

Sometimes I use the Hyperclient::Resource#[] as a hash, but I would like to avoid if statements when the attribute does not exist for some reason.

Did you consider implement the method fetch with similar behaviour of Hash#fetch in ruby?

Thanks!

[alabeduarte]

Just sent a pull request.

[dblock]

Closed via #90.