Briefly describe major objects by providing __str__() functions #313

I'm a newbie to stix, just trying to parse a stix file I ran across. It would be much easier to explore the file, the format and the API if the objects would describe themselves when I print them, rather than just getting something like <stix.indicator.indicator.Indicator at 0x7f80481c0050>

Perhaps the output could look something like these:

Malicious FQDN Indicator: example.com
Malicious IPv4 Indicator (IP Watchlist in Netherlands): 183.81.201.105

Add types and/or ids if they seem useful enough.

Providing a summary description of a whole package, e.g. via some sort of package.summary() function that summarizes the numbers of different types of objects or something like that, would also be handy.