Update installation docs
Update installation docs

Include conda and virtualenv info for pip, add instructions for
downloading models manually and add details and fab commands to
"Compile from source" section.

explosion/spaCy,raphael0202/spaCy,spacy-io/spaCy,oroszgy/spaCy.hu,oroszgy/spaCy.hu,recognai/spaCy,explosion/spaCy,spacy-io/spaCy,raphael0202/spaCy,Gregory-Howard/spaCy,explosion/spaCy,recognai/spaCy,Gregory-Howard/spaCy,Gregory-Howard/spaCy,oroszgy/spaCy.hu,aikramer2/spaCy,honnibal/spaCy,spacy-io/spaCy,spacy-io/spaCy,honnibal/spaCy,oroszgy/spaCy.hu,recognai/spaCy,raphael0202/spaCy,aikramer2/spaCy,aikramer2/spaCy,raphael0202/spaCy,Gregory-Howard/spaCy,explosion/spaCy,raphael0202/spaCy,aikramer2/spaCy,oroszgy/spaCy.hu,recognai/spaCy,honnibal/spaCy,spacy-io/spaCy,spacy-io/spaCy,explosion/spaCy,explosion/spaCy,aikramer2/spaCy,raphael0202/spaCy,Gregory-Howard/spaCy,Gregory-Howard/spaCy,oroszgy/spaCy.hu,honnibal/spaCy,aikramer2/spaCy,recognai/spaCy,recognai/spaCy