move scalacheck-binding into own package: `spire.laws`
remove unneeded entries from .gitignore
tweak build: update SBT, disable publishing for some modules
add `unidoc` plugin

Weirdly, the plugin triggers a bug in scaladoc:

[error] spire/core/src/main/scala/spire/math/Complex.scala:39: recursive method bigDecimalToComplex needs result type
[error]     bigDecimalToComplex(BigDecimal(n))
[error]     ^

Hence, I added type annotations, and everything is fine again.
Set Travis build version to 2.10.2
sbt-release plugin & its friends