Allow consumers to specify a serialization scheme.

Serialization schemes are a convenient way for consumers to add
deserialization logic right in the spout. Add the ability to set a scheme 
through the constructor.
Could you please bump the version back down to `0.3-SNAPSHOT`? 
I see no reason to put the added feature into separate version; let's just ship this with 0.3 when we decide it's time for a new version :smiley: