Adjust EctoFields.Atom to only dump valid strings and atoms.

[potatosalad]

Based on the comment I made about using this for labs, I made the following changes to EctoFields.Atom:

• Adjusted cast/1 to only attempt casting strings that are less than or equal to 255 bytes (maximum length of an atom).
• Changed load/1 to simply be an alias to cast/1.
• Adjusted dump/1 to dump both atoms and strings that are less than or equal to 255 bytes.
• Added simple property test using PropEr.
• Formatted any files I touched.

[jerel]

Thanks for the PR!