added YAMLLoader conditional processing via "if" directive

this PR adds support to conditionally load bundles from a YAML configuration based on environment config values (or any python expression), for example:

``` yaml
js-all:
    contents:
        - jquery.js

        # the following asset, "less.min.js", will only be
        # included if both `debug` AND `less_run_in_debug` are
        # true.

        - if: config.debug and config.less_run_in_debug
          contents: 'less.min.js'
```