Improved slice unmarshalling support

[blakepettersson]

• All integer slice types are supported

JSON number arrays can be unmarshaled as int, int8, int16, int32
and int64 slices, as well as uint, uint16, uint32 and
uint64 slices, as well as slices that are pointers to the
aforementioned types.

In the case of uint8 slices, the assumption is that it is unmarshaled
from a base64 encoded string, since that is how a []uint8 is
marshaled with json.Marshal.

• JSON arrays can also be unmarshaled as float32 and float64 slices,
  as well as the pointer variants of those types.

• time.Time slices are also supported, both values and pointers.

This is similar to #73, but covers more types.

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[blakepettersson]

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