Allow user to pass through any javamail settings they wish

[oliyh]

We would like to supply settings such as mail.smtp.timeout which currently isn't possible as only a select few keys are supplied to the javamail api.

This change allows any mail.smtp.* setting to propagate through to the javamail api.

[killme2008]

A great PR, why not merge it and release a new version?

We need mail.smtp.timeout too.

@drewr

[den1k]

I agree, @drewr please!