Add option to use European sparkpost servers

[olivierh65]

Add an option to use domain eu.sparkpost.com (european acounts).
Add also an option to allow Sparkpost tracking. Usefull to track transactional mails.

[simonjohnparker]

Hi, when do you think this PR will get merged as we need to use the EU Sparkpost servers? Thanks.

[nganivet]

@simonjohnparker Not on our priority list for now, specifically as we need some small tweaks to the submitted PR and it has a conflict with the latest version of the extension.

[petergerner]

see my rebased patch for #70

[herbdool]

@nganivet we have a client that requires this too. Could help with getting some version of the PR merged in.

[herbdool]

@petergerner I think the link above is supposed to be #78

[nganivet]

@herbdool Can you switch the Yes/No to a drop-down for endpoint selection? The Yes/No is confusing and does not lend itself to more servers added in the future. A Select control, with the options taken from a constant variable declared at the top of the class would be clearer and more extensible. Thanks.