Plugable storage backends

[ionelmc]

So my plan is to have an entrypoint based api for plugable storages and custom reporting later.

I think that moving the elasticsearch storage to a separate repo is a good way to prove this can work well, also @drebs could own that code since he contributed it.

I'm thinking about this sort of api:

    entry_points={
        'pytest_benchmark': [
            'storage-uri-example = pytest_benchmark_elasticsearch:uri_example',
            'storage-uri-prefix = pytest_benchmark_elasticsearch:uri_prefix',
            'storage-uri-parse = pytest_benchmark_elasticsearch:uri_parse',
            'cli-add-options = pytest_benchmark_elasticsearch:add_options',
        ],
    }