Yax

[tpolecat]

This rebases and subsumes #304 … original comments below:

---

This adds support for two FP environments by building doobie natively for both. This means there are no shims or indirection; it's distinct builds.

• scalaz 7.2 with scalaz-stream 0.8 for 2.10, 2.11, 2.12
• cats 0.6 with fs2 0.9 (M5) for 2.11 only (no fs2 for 2.10, no cats for 2.12)

This is accomplished via a preprocessor called yax that provides #ifdef-style switches. It's very primitive. The high points:

• All source is now under yax/.
• There are now two versions of each sbt project: core, core_cats, and so on. They have no source of their own and don't appear in source control, but they're generated when you run sbt and you see them under modules/ and modules-cats/.
• Line numbers in errors are wrong. There's a fix for that but it messes up tut so it needs more work.
• cats artifact names are the same but end in -cats, so doobie-core-cats, doobie-h2-cats, and so on.

Unrelated changes in this PR (sorry):

• The contrib segment in artifacts and package names is gone. So doobie-h2 is the artifact now and doobie.h2 is the package name.
• The posgresql segment and package name has been shortened to postgres.

👎 still need

• document module/package/artifact renaming
• get 2.10 and 2.12 travis builds working for scalaz (cats + fs2 is 2.11 only for now)
• get publish to understand different cross-versions
• update readme

[tpolecat]

Ok let's do this.