Improve tokenization of version strings

Tokens like "b4", "beta1", "p195", &c. are now treated as atoms rather
than being broken down even further. Additionally, we enable support for
padding in the middle of versions strings, so we can successfully
compare something like "2.1-p195" with "2.1.0-p194" by inferring that
"2.1" is really "2.1.0".

This fixes the comparison "9.9.3-P1" > "9.9.3" which previously has not
been handled correctly.
omniorb: remove variable
Define to_s for Version::Token
python: Remove duplicate code
PythonInstalled: Don't require superenv

* The python do ... end block does no longer
  require 'superenv' but saves and restores
  the ENV by using ENV.to_hash and `ensure`.
  This should resolve some build problems
  with formulae using `env :std`!
* `python_helper` is now in a module `Python`.
* Imporoved some comments in python_dependency.rb
python: --HEAD option added and more robust builds

* Added a head for python 2.7
* Using `build.with?` instead of `build.include? 'with...'`
* More robust temporary linking of opt/python/bin during
  install and cleanup (with ensure).
Hotfix: Revert Python module
depends_on :python don't set any LDFLAGS

Setting LDFLAGS to `-F HOMEBREW_PREFIX/opt/python/Frameworks`
breaks at least the scipy build. Perhaps others, too.