Support for Virtual filesystem endpoint with documentation.

[diethard-at-alation]

@ignaszewskir I had a quick look at the code:

• In the documentation, under examples, you use alation.virtual_datasource.post_metadata(fs_id=fs_id, vfs_objects=[]) instead of alation.virtual_filesystem.post_metadata(fs_id=fs_id, vfs_objects=[]). Is this on purpose?
• When I tested the code, I couldn't get alation.virtual_filesystem.post_metadata(fs_id=fs_id, vfs_objects=[]) working (to delete the existing objects. It seems like it's not actually calling post because there are no records in the payload?

[ignaszewskir]

@diethard-at-alation Thanks for catching the typo Diethard. On the subject of passing empty payloads, we need to devise an exception in handling this.

[ignaszewskir]

@diethard-at-alation I corrected the readme file and added logic to handle empty object list payloads for delete operations.