Add support for multiple ES nodes

[jenkin]

In settings you can set ELASTICSEARCH_SERVER only by using a simple string. Pyes supports multiple hosts in ES initialization and this string is passed wrapped in a single-element list (see. L50 in scrapyelasticsearch.py).

It could be useful to set ELASTICSEARCH_SERVER also by using a list of hosts, maybe checking if the setting is actually a string or a list, ie via isistance().

[jayzeng]

@jenkin good idea, it is included in #21 (work in progress)

[jayzeng]

Now available at https://pypi.python.org/pypi/ScrapyElasticSearch/0.7