Fix error compiling tests.
Add custom marshaller/unmarshaller for custom types.
Merge pull request #3
rename to Marshaler and use MarshalCassandra suffix
Add `marshal:"json"` struct tag helper

This change adds the ability to register gossie.Marshaler wrappers that will be
automatically used for the given field. Currently, the only
implementation is for `json` but other good options would be
a comma-separated type for string lists, etc.

Example:

    type Round struct {
        ID string `cf:"rounds" key:"ID" json:"id"`
        Bets []*Bet `name:"bets" marshal:"json" json:"bets"`
    }