Support time.Time instead of int64 for date fields

[mbroshi-stripe]

Why?

This change supports codegen changes that converts datetime fields from int64 to time.Time as brought up in this issue: #1205.

What?

• The main change is replacing int64 with time.Time for time fields
• To support this, form serialization in form.go for time.Time is performed by reflection, since we do not want the default behavior of inspecting into the time.Time struct, rather we just convert it to an int64
• While I was in there, I also created convenience methods Time, TimeValue, UnixTime and UnixTimeValue to help in creating time.Time and *time.Time parameter fields
• I also updated unit tests expecting an int64 type

See Also

• Parse unix timestamps into times (time.Time) #1205

[helenye-stripe]

:lurk: I think the merge-base for this PR should be the beta branch!

[jar-stripe]

LGTM!