Initial blueprint for general packet objects

Set up basic fields required for all packet types. Also added accessors
and mutators for these attributes.
Quick indentation fix
Added unit tests for packet class.

Checks correctness of accessor and mutator methods.
Renamed files according to style guide
Added check to mutator
Indentation nit
Added newlines after some docstrings
Added timestamp attribute to packets
Merge pull request #4 from kexinrong/Packet

Initial blueprint for general packet objects
Upload architecture presentation
Merge branch 'master' of https://github.com/kexinrong/NetworkSimulator into Input
Changed case_i to CASE_i