An ebuild for p4est [1]

This commit introduces an ebuild for the p4est library.

[1] http://www.p4est.org/

 "The p4est software library enables the dynamic management of a
  collection of adaptive octrees, conveniently called a forest of octrees.
  p4est is designed to work in parallel and scale to hundreds of thousands
  of processor cores."
Rewrite the ebuild

  - uses autotools-utils, now
  - introduce a static-libs use flag
  - introduce doc and examples use flags that install corresponding files
    directly from the tarball
Remove files directory, the patch is no longer needed
Cleanup as discussed
The restriction on a specific AUTOMAKE version is not necessary
Bugfix: Reintroduce mpi support
Improve the ebuild a bit more

 - Get RDEPEND/DEPEND right
 - omit superfluous "${S}"
Patch the build system to use libtool properly
p4est configured with --enable-mpiio does indeed depend on virtual/mpi[romio]
Install the examples properly to usr/share/doc/${PF}/examples
Merge pull request #95 from tamiko/package-p4est

An ebuild for p4est [1]