Introduce helper function class

[jhodges10]

Changes:

• Introduce helper class and add client.helpers.get_updated_assets() function, replacing Add convenience method for fetching a Projects most recently added assets #53.
• Add a replace option to the download() function to decide whether or not to replace a file during download
• Add client.assets.create_folder(parent_asset_id, name) function to make it easier to create a folder

Fixes:

• Fix missing import for PresentationException in client.py

Needs testing:

• The new `download(asset, replace=True) call has not been tested