Initial stab at SimpleHTTPResolver Functionality

This pull request adds a SimpleHTTPResolver with various common config
options. These options are as follows:

    The config dictionary MUST contain
     * `cache_root`, which is the absolute path to the directory where
        source images should be cached.

    The config dictionary MAY contain
     * `source_prefix`, the url up to the identifier.
     * `source_suffix`, the url after the identifier (if applicable).
     * `default_format`, the format of images (will use content-type
        of response if not specified and no file extension exists).
     * `head_resolvable` with value True, whether to make HEAD requests
        to verify object existence (don't set if using Fedora Commons
        prior to 3.8).
     * `uri_resolvable` with value True, allows one to use full uri's
        to resolve to an image.
     * `user`, the username to make the HTTP request as if needed.
     * `pw`, the password to make the HTTP request as if needed.

Additionally, there is a new clean script titled
"loris-http_cache_clean.sh" in the bin directory with more granular
directory options.

Lastly: it now has a dependency of having the requests library installed
(pip install requests).

Known issues:

1. It still does a base bare identifier call that usually will return
   'false' first before trying more common routing cases.

2. The code to determine an identifier will strip out a slash in
   "http://". I currently compensate for this flaw but should bbe fixed
   in webapp.py's _dissect_uri.

3. Unit test coverage is sparse and needs to be expanded for more
   cases. Additionally, things need to be more properly mocked out
   as using a live url for the resolver at the moment.

4. The "links" and "info" directories are a mess regardless of
   resolver due to all entries not being subdivided. Not sure if this
   matters. In addition, it will write ":" if part of the identifier
   (not allowed on some file systems) along with symlinks being
   unsupported in some drive storage cases.
For all of these if/elses that default to `''` or `None` I think you could do, e.g.

```python
self.source_prefix = self.config.get('source_prefix', '')
```
where the default value is the second param passed to `#get`.