fix parsing of B.C. years

Add small example to documentation for NominalDiffTime.

haddocks: Example how to obtain POSIX int timestamps.

POSIX: Fix haddock links

This also makes it immediately obvious how one can create a value of the type
out of thin air without crawling through the source code of the type class
instances.

You can't link to `System.Posix.EpochTime` since that module isn't imported, haddock will link it into nirvana.

`Foreign.C.CTime` didn't work because that just leads to e.g.
  http://hackage.haskell.org/package/base-4.11.1.0/docs/Foreign-C.html#v:CTime
which doesn't contain the function.