add IPAddressField, update docs

[Ins1ne]

for this #1853

[lovelydinosaur]

Looks good! I could do with some time to review the implementation fully, simply to make sure if there are any clean-ups we can make or not, but everything else seems great.

[LilyFirefly]

This check is unnecessary. '' will always just pass though to the final return.

[lovelydinosaur]

Going to drop the 3.1 marker on this so that I make sure to take the time to review properly. (We're about to release tomorrow, so...)

[andreagrandi]

I may be wrong, but this line looks like we assume the user will always use 'both' protocol, but what happen if we can't unpack the ipv4 address to an ipv6 one? In this case ':' won't be present inside the ip string and the whole check will fail or be skipped.

Is this situation already handled by the use of ip_address_validators method? Should we add a particular test case that handle the situation where the user pass (protocol='ipv4', unpack_ipv4=True) ?

[lovelydinosaur]

Sounds right!

[jpadilla]

Closing this in favor of #2747