Support timestamp type in different units

[federicojasson]

The current implementation uses ns as a fixed unit for timestamp fields: https://github.com/andrewgross/json2parquet/blob/master/json2parquet/client.py#L98

When using pa.Schema, the unit is ignored:

schema = pa.schema([
    pa.field('a', pa.int64()),
    pa.field('b', pa.string()),
    pa.field('c', pa.float64()),
    pa.field('d', pa.timestamp("s"))
])