Call callback only once (even on error)

[maxkoryukov]

fix #46

based on blogpost: http://joseoncode.com/2013/12/27/case-of-double-callbacks/

[coveralls]

Coverage remained the same at 100.0% when pulling 7b6ea37 on maxkoryukov:fix/callback-doublecall into e95095d on A:master.

[tbranyen]

Looks good to me!

[tbranyen]

Thanks for the fix @maxkoryukov

[maxkoryukov]

@tbranyen , plz, tell me, when the package on NPM will be updated? I want to switch from git-dependency to NPM-dep (you know - it is much more convenient)

[tbranyen]

I understand, I'll try and push it out tomorrow.

[maxkoryukov]

@tbranyen , NPM still keeping 0.5.2... 😢

[maxkoryukov]

@tbranyen , LOOK MA)) still 0.5.2 👐

[maxkoryukov]

Hello 🚶‍♂️ Are there anybody out there;) @tbranyen , plz upgrade NPM