Reloading collections with Zookeeper

[flyingzumwalt]

includes changes from #14

• Extracts exec command into a CommandLineWrapper class
• Refactors CommandLineWrapper .exec to support switching executables (ie. solr binary & zkcli)
• Adds exec_solr and exec_zookeeper convenience methods to SolrInstance objects
• Adds methods for uploading collection configs to zookeeper and reloading collections -- this is so that you can update collection configs without having to delete the whole collection, re-create it and re-import its data.