Add log to dplyr function ungroup()

[damianooldoni]

This PR aims to solve #32.

1. I opted for log test: ungroup: no grouping variables left.
2. New R file ungroup.R added to ./R directory. Another way to work would be to add function ungroup() in file ./R/group_by.R.
3. I added a related test file: test_ungroup.R
4. I slightly improved other tests as well. For example: no need to load library in each test file.
5. README is also updated by adding ungroup() at the end of example about grouping.

Travis says yes and pkg documentation seems ok 👍
Have a deep look and let me know. Especially point 2 seems something you, as maintainer, has the last word about.

Thanks for writing, updating and maintaining this wonderful package.

[elbersb]

Thanks!
I added a few comments. As you've suggested I'd prefer the ungroup function be part of group_by. (Same for the tests)
As for the tests files, I actually like having the library calls on top, because that makes debugging them easier. Could you revert? In general, also best to keep things in separate PRs, so it's easier to review :)
Also feel free to add a new bullet point in NEWS.md.

[elbersb]

not a typo

[damianooldoni]

Done: 3e70eea

[elbersb]

please use just "ctb" here

[damianooldoni]

I am very sorry: I am definitely not the author! A copy paste error.
Solved here: baae929

[elbersb]

please leave as is, or change to "second"

[damianooldoni]

Yes, you are right: it refers to the fact that there are two filters, indeed.
Solved here: 39c624f

[damianooldoni]

I think I solved all requested changes. Please, give a look again. Thanks.

[elbersb]

Thanks, that's great.
Unfortunately, I had just had another idea, and that is to simply add this functionality to the existing group_by function. This has two advantages: 1) it takes care of the case group_by(mtcars), which prints something ugly right now, and 2) makes the package more future proof in case the ... argument in ungroup is ever taken seriously. (see, e.g., tidyverse/dplyr#3760)

If you'd like to have another go at this, feel free -- if not, I'll merge and then implement this.

[damianooldoni]

I like your idea. I gave it a try. Let me know. Thanks.

[elbersb]

Thanks!