Fix sporadic test failure in test_plugin_cycle

Triggered by plugin installation taking longer than the timeout on
popMessage()
Fix make_ssl_certificate on Python 2

Fixes the following exception:

Traceback (most recent call last):

File "/home/travis/build/zoni/err/errbot/backends/base.py", line 520, in
_execute_and_send

for reply in replies:

    File "/home/travis/build/zoni/err/errbot/builtins/webserver.py",
    line 185, in generate_certificate

    make_ssl_certificate(key_path=key_path, cert_path=cert_path)

    File "/home/travis/build/zoni/err/errbot/builtins/webserver.py",
    line 58, in make_ssl_certificate

    cert.sign(pkey, u'sha256')

    File "build/bdist.linux-x86_64/egg/OpenSSL/crypto.py", line 827, in
    sign

    evp_md = _lib.EVP_get_digestbyname(_byte_string(digest))

    TypeError: initializer for ctype 'char *' must be a str or list or
    tuple, not unicode
Test Python 3.4 with Travis as well

Err supports Python 3.3+, so best to test all Python versions Travis
supports, just in case there is ever a difference between the various
3.x versions that causes incompatibilities.
Add requirements-development.txt

This makes it easy to install all development-related packages without
having to specify them manually in multiple places.
Call pep8 directly from run_tests.py
Update .gitignore
Exclude ./build from pep8 checks
Docs: Also autodoc __init__ methods
Allow webserver_webhook_test to be run parallel

It was working previously because of the order of tests coincidentally
configuring this plugin in an earlier-run test.