Add refresh image button

[freemanoid]

Every captcha have a possibility to update the current picture by clicking on it our via special refresh button.

I have added the refresh button to simple_captcha.
The one current requirement is that we need to have jquery_ujs on the page with captcha. I don't know how to include it via simple-captcha correctly (we need to include jquery_ujs if it hasn't included on this page already)

[zealot128]

thanks @freemanoid for the contribution, that would be quite an improvement.

I started to add some basic tests for simple captcha. Do you think, it will be possible, to add some basic tests that cover the new functionality?
Maybe we will just tell in the README that jquery_ujs is required. Maybe later will can make a switch/config option for setting the Refresh button. What do you think?

[freemanoid]

I'll add some tests and attach to this issue when it will be done.

[zealot128]

Thanks for all the ground work. I merged the changes and made the tests pass, which fail because of a missing session. Besides, I tried to generalize the captcha image id because of #8.
It is on master now, but I didn't released a new gem yet. If you have some time, can you try this new release with your app?