Update "githug reset" function

[zhanglintc]

Now githug reset only accept level name, such as githug reset init means change to the first level.

But I just want to make it accept numbers, such as githug reset 1 to jump to the 1st level. I think it would be more convenient. That's why I made this pull request.

If you think this is good please accept this pull request. Thanks.

[Gazler]

Thanks for this!

Could you add a test similar to https://github.com/zhanglintc/githug/blob/81a1c16bf391e09dbf94934591a94105708f96ab/spec/githug/cli_spec.rb#L101

Could you also update the docs at https://github.com/zhanglintc/githug/blob/81a1c16bf391e09dbf94934591a94105708f96ab/lib/githug/cli.rb#L41

To be:

      > $ githug reset merge_squash  # or $ githug reset 45

After that I'll merge this in.

[zhanglintc]

OK. I have updated the docs and added a new test. But I'm not sure if the test I've added is right or not because I'm not familiar with rspec. Please double check it.

I don't know why Ruby 1.8.7 and Ruby 1.9.2 are always failed in Travis CI build. I think it has no relation to my code.

[Gazler]

Thanks!

I need to work out why 1.8.7 and 1.9.2 are failing, it is unrelated to your changes though.