Fix for S1lentium/IPTools#15

[petski]

IPv6 blocksize can exceed the maximum number for integer (2^32 or 2^64).

This commit fixes $network->count() [1] (by returning the number of hosts as a string if it exceeds the max integer size). It can't fix count($network) [2], since PHP's count() must return an integer.

[1] Or $range->count()
[2] Or count($range)

[coveralls]

Coverage remained the same at 96.491% when pulling 399b553 on petski:master into 5ce333e on S1lentium:master.

[S1lentium]

Yes, this fix. But it breaks concept of the Countable interface. Or you can delete this interface, and check count methods on other classes.

[petski]

It's not that my patch breaks anything. My patch actually makes the issue described in this bug-report work for $network->count(). The issue indeed remains if the Countable interface count($network) is used, due to limitation of PHPs count() method...

I agree that dropping the Countable interface is probably the best idea. Unfortunately, that will break backwards compatibility. Users should be informed about this in the upcoming release.

Let me know if you indeed want this approach. I will update my patch accordingly (or you may do so, if you want)