Replace `poly2d` class with `polyval2d()`

Prior to `numpy<1.4.0`, only `numpy.poly1d` class provided functionality
dealing with polynomials.  Hence, it was a reasonable choice for me to
implement `polynomial2d.poly2d` as a class after `numpy.poly1d` class.
However, since `numpy>=1.4.0`, the functionality provided by
`numpy.poly1d` class has been superseded by a newer polynomial package
`numpy.polynomial` and by its module `numpy.polynomial.polynomial`.
Moreover, since `numpy>=1.7.0`, the module added functions for
evaluation of multivariate polynomials (or 2D or 3D polynomials), namely
`polyval2d()` and `polyval3d()`.

Thus, I have decided to provide a function for evaluation of 2D
polynomials that simply wraps `numpy.polynomial.polynomial.polyval2d()`.
Aside from that the functionality is now provided by a function instead
of a class, the major change from the previous interface is that
polynomial coefficient must be given by a 2D array *NOT* a 1D array.  I
hope this change will enhance interoperability with `numpy`.
Use absolute_import
Improve Python3 compatibility
Remove ncoeffs() and order()
Make polyfit2d() compatible with numpy.polynomial

Previously, the 2-dim polynomial fitting routine in this package
returned polynomial coefficients as an 1-dim array.  However, since its
version 1.7.0, NumPy added functions that evaluate 2-dim polynomials
(`numpy.polynomial.polyval2d()`, etc.).  Those functions expect that
polynomial coefficients are given as a 2-dim array, satisfying::

    p(x, y) = sum[i=0:n](sum[j=0:n] c_ij * x^i * y^j)

Thus, the polynomial coefficients returned by the fitting routine have
been incompatible with the relevant NumPy functions.

To fix the incompatibility, I implemented a 2-dim polynomial fitting
routine `polyfit2d()`.  The function assumes the 2-dim polynomial in a
form expressed as::

    p(x, y) = sum[i=0:n](sum[j=0:n] c_ij * x^i * y^j)

with a constraint of ``i + j <= n`` and returns the polynomial
coefficients `c` (`c_ij`) as a 2-dim array.  Note that, due to the
constraint, the elements in the lower right triangle of `c` are zeros.
Improve doc
Use lowercases for the package name
Drop an extension module _poly2d

The extension module `_poly2d` was originally introduced to accelerate
computation of 2-dim polynomials. The functions in the module expect the
coefficients of a polynomial in an 1-dim array.  However, that form is
incompatible with newly introduced `polyval2d()`, which expects the
coefficients in a 2-dim array.  In addition, I suspected that the
benefit of the acceleration is marginal.  Thus, I have decided to
entirely drop `_poly2d`.