Fixes GH417

New top level :py:func:`merge` function allows for combining variables from
any number of ``Dataset`` and/or ``DataArray`` variables.

Example usage:

    >>> arrays = [xr.DataArray(n, name='var%d' % n) for n in range(5)]
    >>> xr.merge(arrays)
    <xarray.Dataset>
    Dimensions:  ()
    Coordinates:
        *empty*
    Data variables:
        var0     int64 0
        var1     int64 1
        var2     int64 2
        var3     int64 3
        var4     int64 4

The internal refactoring also lays the ground work for supporting ufunc-like
functions that merge three or more arguments, such as the full form of
`where`.