Add coveralls support

[vanlooverenkoen]

No description provided.

[dazzlingy1212]

I am confused about a qusetion , at the file certificate_util.dart,line 36 is final der = base64Decode(pem) , so what need to declare a variable cert in the method extractKid ?
Also when I use it, that show the error about invalid character at there. I change the variable pem to cert,and it is without error.
Can anybody tell me is it right?

[jeroentrappers]

To get the kid, you pass in the Document Signing Certificate. This can be in PEM or DER format. The function will strip the header and footer in case you pass PEM format. The DER and PEM format differ only in these headers and footer.

Then it takes the first 8 bytes.