add int rule

[jakesylvestre]

As per some pr's that have been sitting around for a while (e.g. #93) and uncertainty as to how this framework is used in the larger ozzo project I wanted to make sure that this repo was still open before I spend too much time on this.

I've taken the int_rule functionality (and corresponding test case) I added in ozzo-validators and added it here. This allows you to validate structs on non-string fields.

For instance, IsValidPort and IsAvailablePort here validates on an int rather than a string.

Here's what that looks like in the context of a struct validation:

[coveralls]

Coverage decreased (-0.2%) to 99.225% when pulling 393bfcd on xplorfin:int-rule into 34bd547 on go-ozzo:master.