Pull-request for issue-10

[mikhail-akimov]

Added the ability to receive a short version of the commit.
Closes #10

[coveralls]

Coverage remained the same at 100.0% when pulling 90a10a6 on mikhail-akimov:Issue-8 into 23172f3 on sobolevn:master.

[mikhail-akimov]

I think it's not compleate. Let me tune it.

[sobolevn]

Thanks for putting this together! Some changes are required.

[mikhail-akimov]

I getting short=False twice (1-st in parse() and second at _commit_hash() as default value). Can i optimize it somehow?

[sobolevn]

@mikhail-akimov remove the default value from _commit_hash function.

[sobolevn]

@mikhail-akimov awesome work, thanks!