new configuration option "webassets.paths" for multiple asset locations

currently, if your assets are distributed in multiple directories (say "/path/to/asset" and "/other/assets"), you need to write python code:

``` python
env = config.get_webassets_env()
env.append_path('/other/assets', '/other')
```

in addition to the configuration:

``` ini
webassets.base_dir = /path/to/asset
webassets.base_url = /static
```

which means that there are two places that asset locations get defined, which is non-ideal.

this PR adds a new configuration option ``webassets.paths`` which is a JSON structure that maps directories to exposed URLs (the latter being optional).