Add @JsonAlias annotation to allow specifying alternate names for a property

[cowtowncoder]

(note: pre-requisite for FasterXML/jackson-databind#1029)

We need a new annotation for specifying aliases, as per issue linked to above.

An alternative would be to add a new property (f.ex to @JsonProperty), but it seems better to add separate one since these are most likely generated by tools.