Add support for retrieving conversation tags

[thewheat]

Addressing #128

• uses getTagCollection instead of suggested getTags for consistency with other existing calls on TagCollection

intercom-java/src/main/java/io/intercom/api/Company.java:    public TagCollection getTagCollection() {
intercom-java/src/main/java/io/intercom/api/Contact.java:    public TagCollection getTagCollection() {
intercom-java/src/main/java/io/intercom/api/User.java:    public TagCollection getTagCollection() {

• Code to retrieved tags modelled after getConversationPartCollection

  intercom-java/intercom-java/src/main/java/io/intercom/api/Conversation.java

  Lines 267 to 273 in aa17143

  	public ConversationPartCollection getConversationPartCollection() {
  	if (conversationPartCollection == null) {
  	conversationPartCollection = find(this.getId()).getConversationPartCollection();
  	}
  	return conversationPartCollection;
  	}

Sample test run using the new code

[choran]

I thought initially this was an issue with potential infinite loop but actually the opposite. Mentioned it to @mmartinic and he noted that this is used for list as well as single convos so there could be a case where there are no tags returned for listing class.

[choran]

Yeah, nice work, good catch with the check for no tags.
👍