Feature: plugins detection using entrypoints

[sijis]

This feature should allow plugins to be detected using entrypoints when installed via pip.

The entrypoints are

• errbot.plugins - normal botcmd plugins
• errbot.backend_plugins - plugins related to communications systems (Slack, Gitter, etc)

A package would need to include this snippet in their setup.py to work. Using, https://github.com/errbotio/err-helloworld as an example, we need to add

...
    entry_points = {
        "errbot.plugins": [
            "helloworld = helloWorld:HelloWorld",
        ]
    }
...

I would suggest, plugins use the naming convention of errbot-plugin-$pluginName