[MRG] cythonize fill_hist and link everything into one library

* cythonize fill_hist and add a new return_indices argument:

```python
def fill_hist(hist, array, weights=None, return_indices=False):
    """ 
    Fill a ROOT histogram with a NumPy array.

    Parameters
    ----------

    hist : a ROOT TH1, TH2, or TH3
        The ROOT histogram to fill.

    array : numpy array of shape [n_samples, n_dimensions]
        The values to fill the histogram with. The number of columns
        must match the dimensionality of the histogram. Supply a flat
        numpy array when filling a 1D histogram.

    weights : numpy array
        A flat numpy array of weights for each sample in ``array``.

    return_indices : bool, optional (default=False)
        If True then return an array of the bin indices filled for each element
        in ``array``.

    Returns
    -------

    indices : numpy array or None
        If ``return_indices`` is True, then return an array of the bin indices
        filled for each element in ``array`` otherwise return None.

    """
```

* reorganize pyx, and link everything into one library.
* and a new utility in the Makefile for creating a viewing the html annotation:

```
> make show-cython
compiling root_numpy/src/array.pyx ...
compiling root_numpy/src/hist.pyx ...
compiling root_numpy/src/innerjoin.pyx ...
compiling root_numpy/src/matrix.pyx ...
compiling root_numpy/src/sample.pyx ...
compiling root_numpy/src/tree.pyx ...
opening root_numpy/src/array.html ...
opening root_numpy/src/hist.html ...
opening root_numpy/src/innerjoin.html ...
opening root_numpy/src/matrix.html ...
opening root_numpy/src/sample.html ...
opening root_numpy/src/tree.html ...
```

This is required since cython doesn't annotate included files and `_librootnumpy.pyx` is now just this:

```
# cython: experimental_cpp_class_def=True
include "setup.pxi"
include "tree.pyx"
include "array.pyx"
include "matrix.pyx"
include "sample.pyx"
include "hist.pyx"
include "innerjoin.pyx"
```