Conda build now checks for runtime requirements file

With this pull request conda will check for a `requirements.txt` file beside the `meta.yaml` file in the recipe directory. If found the `requirements.run` will be set to the contents of `requirements.txt`

This is so that developers can run: 

    conda install --file conda.recipe/requirements.txt
    python setup.py develop

To develop a conda package locally.  When you want to build a package, the command is still:

    conda build conda.recipe