Add constructor parameter to File API for loading the metadata on intialization

[manoss96]

Instead of the user having to do the following in order to access the file metadata:

from fluke.storage import AmazonS3File

with AmazonS3File(auth, bucket, path) as f:
    f.load_metadata()
    meta = f.get_metadata()

They should be able to just do:

from fluke.storage import AmazonS3File

with AmazonS3File(auth, bucket, path, load_metadata=True) as f:
    meta = f.get_metadata()