Add GIT_ environment variables to conda-build

Adds the following new environment variables when building:

* ~~`GIT_BUILD` -- boolean denoting whether a git_url has been used (detects via presence of `.git` directory in `SRC_DIR`~~
* `GIT_DESCRIBE_VERSION` -- string denoting the version based solely on presence of a tag
* `GIT_DESCRIBE_NUMBER` -- string denoting the number of commits since the most recent tag
* `GIT_DESCRIBE_HASH` -- the current commit short-hash as displayed from `git describe --tags`
* `GIT_BUILD_STR` -- a string that joins `GIT_BUILD_NUMBER` and `GIT_BUILD_HASH` by an underscore.

These can be used in conjunction with templated `meta.yaml` files to set things like the build string based on the state of the Git repository.

For example, here's a modified wakari-app-workbench `meta.yaml` that would work with these values:

    package:
      name: wakari-app-workbench
      version: {{ environ['GIT_BUILD_VERSION'] }}

    build:
      number: {{ environ.get('GIT_BUILD_NUMBER', 0) }}
      string: {{ environ.get('GIT_BUILD_STR', '') }}

      entry_points:
        - wk-app-workbench = workbench.server:main

    source:
      git_url: ../


Yup -- otherwise we have to open up two processes, one to see if there are any tags and another to do the actual description.