Make `conda build` R-aware.
Checkpoint commit of CRAN/R package work.

Main changes:

    - Support multiple CRAN sites, not just the cran.r-project.org one.
      (We currently support cran, bioconductor, bioconductor_annotation,
       and omegahat.)

    - Track forward and reverse dependencies and ignore packages with
      unsatisfiable dependencies.

    - Introduce new 'builddata.json' metafile that is analogous to
      'repodata.json'; where the latter focuses on the relationships
      already built packages, the former allows us to define said
      relationships between possibly-yet-to-be-built packages.

      This will allow conda to be enhanced such that it can either:
        a.) build all packages rooted in a directory (like our
            cran-conda-recipes repo) that has a builddata.json
            (including automatic dependency resolution and ordering)

        b.) automatically attempt to build all dependent packages for
            a given package first (rather than requiring the packages
            to already be installed/built/available-from-binstar)