Email adapter have different blocking behavior

[Musashi178]

Hi,

I noticed that the TestAdapter will do a non-blocking cast for delivering emails, but other adapters will do a blocking call. This causes different handling of the deliver result depending whether I am running my tests or being in dev or production mode. They all should be either blocking or non-blocking.

[skosch]

In fact, I think it would be nice if we offered more flexibility, similar to what thoughbot's bamboo library is doing (blocking, non-blocking, optional delays, etc. are all useful to have depending on the situation).