appveyor badge is broken

[mdreizin]

@gajus You have refactored code. I liked style=flat-square, but now it uses wrong url:

It should use:

When you have travis and appveyor it is difficult to detect them, because they have the same picture:

It would be nice to add logo for appveyor:

or add label

[gajus]

@mdreizin Is this error still present?

[mdreizin]

@gajus Yes, because #43 has not been implemented.