FileSystemResourceManager changes

- [x] Allow aliases in manager config
- [x] Added `override` option, to prevent rewriting a file
Merge pull request #3 from slavcodev/patch-1

FileSystemResourceManager changes
Added listFiles() and $expires to getUrl()

Added expires to retrieve signed url (expire after supplied timer
period) to the getUrl method, and added a listFiles method to get a
list of files within a virtual directory provided keys follow pathname
syntax
Minor fixes
Merge pull request #4 from andrewblake1/master

Thank you... great addition
Fixes #6
Merge branch 'master' of github.com:2amigos/yii2-resource-manager-component

Conflicts:
	FileSystemResourceManager.php