Add ability to prefer particular versions in packages.

[tgamblin]

This is a stopgap until I can merge PR #120, which includes more robust concretization preferences. This allows any number of versions in a package to be marked preferred=True. Packages are now sorted by preference, then most recent in concretization. So you can mark directly in the package file what the preferred version is.

PR #120 provides the infrastructure we need to express these types of preferences per-platform, per-compiler, etc. but this is a first step. This also allows Python 2.7.11 to remain the default while we get the Python stack working with 3.5.

@lee218llnl @alalazo @mplegendre: What do you think?

• Adding preferred=True to a version directive will change its sort
  order in concretization.
• This provides us a rudimentary ability to keep the Spack stack
  stable as new versions are added.
• Having multiple stacks will come next, but this at least allows us
  to specify default versions of things instead of always taking the
  newest.

[tgamblin]

@lee218llnl: does this look ok to you? This will allow you to prefer python 2.7 until 3 is more stable. Also would allow you to designate "stable" versions of other packages

[alalazo]

@tgamblin : it seems good to me. As you said it will improve stack stability changing only a few lines of code.

[lee218llnl]

@tgamblin this looks good to me

[tgamblin]

Thanks!