Adds reload_collection method

[flyingzumwalt]

This allows you to reload a collection's configs. This is the only reliable way to reload a collection's configs other than destroying the entire solr instance and installing a new instance from scratch. Otherwise, the collection configs stay cached even when you delete the collection and create it again or stop & start solr.

Note: This is just one commit. It comes after #14, which is the other commit you can see on this branch.

[flyingzumwalt]

I'm closing this PR because I realized that the this reload method method doesn't do anything if you created the collection using solr_instance.create(dir:'...'). It's meant for uses where you upload the configs into zookeeper using solr/bin/zkcli, in which case you probably want to do a bunch of other things using zkcli which we don't really want/need solr_wapper to support.

[flyingzumwalt]

For reference, in cases where you created the collection using solr_instance.create(collection_name, dir:'...'), the best way to refresh the configs is to use

solr_instance.update_or_create(collection_name, dir:'...')