Improve package and directory structure

[L-Lavigne]

Our package structure does not match our directory structure, for example in the file src/main/scala/mu/print.scala:

package skeuomorph
package mu

This causes various inconveniences such as ScalaTest/Specs2 integration in IntelliJ IDEA being unable to identify and run tests together from the src/test root.

We also use chained package declarations which is somewhat non-standard, and it might be worth revisiting that choice.

A re-arrangement of packages would also be a good time to change the package roots once we've agreed internally on a naming convention for higherkindness, as part of #18.

Similar suggestion for mu: higherkindness/mu-scala#480.

[JesusMtnez]

@L-Lavigne if I am understanding this correctly, it means moving things to src/main/scala/higherkindness/skeumorph/mu for the example, right?

[L-Lavigne]

That's right, assuming we've agreed on the higherkindness prefix.

As described I'd also like to change the package declarations to the standard style unless we have a good reason to keep the chained style.

[JesusMtnez]

Perfect then. About the package declarations, I agree on using standard style. There is not any special reason, as far as I know.