Improve error message if mongodb is not running

[motine]

Catch the exception and give a reasonable error message with what to do to fix it, instead of this:

Traceback (most recent call last):
  File "src/main.py", line 42, in <module>
    main()
  File "src/main.py", line 21, in main
    pm.init(config.PLUGINS_PATH)
  File "/Users/motine/Documents/AMsoil/dev/Ohouse/src/amsoil/core/pluginmanager.py", line 261, in init
    pluginInfo.setup()
  File "/Users/motine/Documents/AMsoil/dev/Ohouse/src/amsoil/core/pluginmanager.py", line 146, in setup
    self._plugin_module.setup()
  File "/Users/motine/Documents/AMsoil/dev/Ohouse/src/plugins/mongodb/plugin.py", line 5, in setup
    mongo_database = MongoDB()
  File "/Users/motine/Documents/AMsoil/dev/Ohouse/src/plugins/mongodb/mongodatabase.py", line 25, in __init__
    client = pymongo.MongoClient()
  File "/Users/motine/.virtualenvs/ohouse/lib/python2.7/site-packages/pymongo/mongo_client.py", line 352, in __init__
    raise ConnectionFailure(str(e))
pymongo.errors.ConnectionFailure: could not connect to localhost:27017: [Errno 61] Connection refused

[motine]

Also, add info on how to start the mongo server in readme

[motine]

and fix the link to MongoDB in the readme

[motine]

and add the requirement to install mongo (e.g. brew install mongo)