Test multiple directives

[mkelley]

Following #219, this PR adds tests for two directives in series. For example:

.. doctest-requires:: numpy<=0.1
.. deprecated:: 1.0

    >>> 1 + 3
    2

Currently this example will fail building with Sphinx with the same error as shown in #219 (i.e., it was not fixed by #311). The error arises because .. doctest-requires in the above example has no content following it, so indexing self.content in its run() fails:

class DoctestSkipDirective(Directive):
    has_content = True

    def run(self):
        # Check if there is any valid argument, and skip it. Currently only
        # 'win32' is supported.
        if re.match('win32', self.content[0]):
            self.content = self.content[2:]
        code = '\n'.join(self.content)
        return [literal_block(code, code)]

In this PR, I've added a test in the if statement to first require len(self.content) > 0 before testing for "win32", but the rendered documentation does not look good. There are empty code blocks added for each "empty" doctestplus directive:

Recognizing that Sphinx directives in series do have have this problem, I tried sub-classing from SphinxDirective and the result looks good:

I don't know if you want to take that direction, but I figured that this PR could at least show a possible solution.

[pllim]

Looks pretty clean but I'll give others a chance to review. Thanks!

Could use change log in CHANGES.rst.