Adds whitelist option.

You can now set a whitelist of file paths that should be copied to a non digest version.

## Usage example

Add an initializer into `config/initializers` folder:

```
    # config/initializers/non_digest_assets.rb
    NonStupidDigestAssets.whitelist = ['tinymce/']
```
Refactors the whitelist implementation and adds a "exact file match" feature.

1. The whitelisting now happens in the `NonStupidDigestAssets` module.

2. Adds "exact file match" feature:

The whitelist now takes regular expressions and exact file names (including path).

Example:

```
NonStupidDigestAssets.whitelist = [/^tinymce\//, 'select2/license.txt']
```

This would copy all files beginning with `tinymce/` and the `select2/license.txt` file.
All other files will be skipped.
Check for existence before copying assets. Warn if not found.