add consistency to code

[Ironholds]

In particular this:

1. Avoids alternating the level of indentation;
2. Uses a consistent control flow format;
3. Consistently uses <- rather than =
4. Consistently uses explicit return statements, rather than a mix of explicit and implicit.

In addition:

1. importFrom statements are used wherever possible (you're only calling two dplyr functions; loading the whole package is overkill);
2. "depends" is bumped down to R 3.2.0 - the vast vast vast vast majority of users do not yet have 3.2.1 and cutting them out will limit your package's audience unnecessarily. I've tried it on 3.2.0 and it seems to work fine :)

[fsolt]

Thanks again, Oliver!