Add Unique IDs to Envelope's Parts

[philipyaz]

cf. issue "Add Unique ID's to Envelope's Parts" #36

In my proposition, the Parts' PartIDs will look as such:

multipart/alternative                                       PartID = 0 (root)
|-- text/plain                                                    PartID = 1
|-- multipart/related                                        PartID = 2.0 ("multipart/")
|     |-- text/html                                               PartID = 2.1
|     |-- image/png, disposition: inline, filename: "img3.png"   PartID = 2.2
|     |-- multipart/related                                  PartID = 2.3.0 ("multipart/")
|     | `-- text/html                                            PartID = 2.3.1
|     `-- image/png, disposition: inline, filename: "img3.png"   PartID = 2.4
`-- image/png, disposition: inline, filename: "img3.png"         PartID = 3

[coveralls]

Coverage increased (+0.3%) to 86.253% when pulling 366219f on philipyaz:feature/PartID into a22a93f on jhillyerd:develop.