added 0.6.0 mongo migration helper, details follow

The clear_evidence_facets_cache.rb mongo migration will clear out new
evidence facet cache storage in case searches were built before
migrating all documents in the "evidence" collection.

tools/migrations/0.6.0/clear_evidence_facets_cache.rb

@@ -0,0 +1,75 @@
+#!/usr/bin/env jruby
+
+# Clears out evidence facet caches that may have been built before evidence
+# documents were migrated for 0.6.0.
+#
+# Mongo migration:
+# - Drops all evidence_facet_cache_* collections.
+# - Removes all documents from evidence_facet_caches that referenced the
+#   dropped collections.
+#
+
+require 'openbel/api/config'
+require 'openbel/api/version'
+
+VERSION_REQUIREMENT = "0.6.0"
+ACTIVE_VERSION      = OpenBEL::Version::STRING
+
+ENV['OPENBEL_API_CONFIG_FILE'] ||= (ARGV.first || ENV['OPENBEL_API_CONFIG_FILE'])
+
+unless ENV['OPENBEL_API_CONFIG_FILE']
+  $stderr.puts "usage: clear_evidence_facets_cache.rb [CONFIG FILE]\n"
+  $stderr.puts "Alternatively set the environment variable OPENBEL_API_CONFIG_FILE"
+  exit 1
+end
+
+def setup_mongo(cfg)
+  require 'mongo'
+
+	host = cfg[:host]
+	port = cfg[:port]
+	db   = Mongo::MongoClient.new(host, port,
+    :op_timeout => 300
+  ).db(cfg[:database])
+
+	# Authenticate user if provided.
+	username = cfg[:username]
+	password = cfg[:password]
+	if username && password
+		auth_db = cfg[:authentication_database] || db
+		db.authenticate(username, password, nil, auth_db)
+	end
+
+  db
+end
+
+def migrate(mongo)
+  if mongo.collection_names.include?('evidence_facet_cache')
+    mongo['evidence_facet_cache'].remove({})
+    puts %Q{Removing documents from "evidence_facet_cache" (success).}
+  end
+
+  mongo.collection_names.select { |name|
+    name =~ /^evidence_facet_cache_[0-9a-f\-]+$/
+  }.each do |name|
+    mongo.drop_collection(name)
+    puts %Q{Dropped "#{name}" collection (success).}
+  end
+
+  true
+end
+
+case ACTIVE_VERSION
+when VERSION_REQUIREMENT
+
+  cfg = OpenBEL::Config.load!
+  migrate(
+    setup_mongo(cfg[:evidence_store][:mongo])
+  )
+  exit 0
+else
+
+  $stderr.puts %Q{Migration is intended for version "#{VERSION_REQUIREMENT}".}
+  $stderr.puts %Q{Version "#{ACTIVE_VERSION}" is currently installed.}
+  exit 1
+end