add support for GET user/activities #248
add support for GET user/activities #248

Since the response of `GET user/activities` contains a date format (`YYYY-MM-DD`) that cannot be unmarshalled into a `time.Time` automatically, additional logic was added to to cover the un-marshalling of the timestamps:

`UserActivityTime.UnmarshalJSON()`
`UserActivityTime.MarshalJSON()`
`UserActivityTime.IsSet()`

see https://stackoverflow.com/questions/25087960/json-unmarshal-time-that-isnt-in-rfc-3339-format

xanzy/go-gitlab,mickymiek/go-gitlab,nyxi/go-gitlab