Support for attributes with different dtypes when serialising to netcdf4

At the moment, bool and dates aren't supported as attributes when serializing to netcdf4:

>>> da = xr.DataArray(range(5), attrs={'test': True})
>>> da 
<xarray.DataArray (dim_0: 5)>
array([0, 1, 2, 3, 4])
Dimensions without coordinates: dim_0
Attributes:
    test:     True

>>> da.to_netcdf('test_bool.nc')
...
TypeError: illegal data type for attribute, must be one of dict_keys(['S1', 'i1', 'u1', 'i2', 'u2', 'i4', 'u4', 'i8', 'u8', 'f4', 'f8']), got b1

>>> da = xr.DataArray(range(5), attrs={'test': pd.to_datetime('now')})
>>> da
<xarray.DataArray (dim_0: 5)>
array([0, 1, 2, 3, 4])
Dimensions without coordinates: dim_0
Attributes:
    test:     2017-08-03 13:02:29

>>> da.to_netcdf('test_dt.nc')
...
TypeError: Invalid value for attr: 2017-08-03 13:02:29 must be a number string, ndarray or a list/tuple of numbers/strings for serialization to netCDF files

I assume bool attributes aren't supported by netcdf4-python and dates are difficult (could always just write these as a string), but this would be really nice to have if possible.

As an aside, using h5netcdf works for bools, but coerces them to int64.

Start at the DataArray.to_netcdf entry point and reproduce the boolean and pandas datetime examples from the issue using the netcdf4 engine. Trace the netCDF4 serialization path and determine the expected representation for each attribute type; done means both examples serialize successfully without the reported TypeErrors.