Review and merge upstream pull requests

[ivy]

The following pull requests need to be reviewed and merged:

• [x] Export NetDialTimeout to public (go-gomail/gomail#100)
• [ ] Added Check For Existing Files And Attachments (go-gomail/gomail#97)
• [x] Add syntax highlighting to Go example (go-gomail/gomail#92)
• [x] ~fix mail attachment name garbled (go-gomail/gomail#83)~ - This one needs more attention to be sure charset encoding is being handled correctly.
• [x] ~Break out SMTP Client Reset Method (go-gomail/gomail#79)~ - Moved to #8
• [x] Accept io.Reader for Attach and Embed (go-gomail/gomail#78)
• [x] ~Added Go 1.7 to .travis.yml (go-gomail/gomail#73 go-gomail/gomail#74)~ - Fixed in 92faf9500adb68b6a67269dc6fe74ce47e3056ac
• [x] Allow timeout in connections, retries to be configurable (go-gomail/gomail#70)
• [x] ~Add support for attaching/embedding files for URLs (go-gomail/gomail#59)~ - I believe this kind of behavior is out of the scope of this project. If anyone disagrees we can discuss more.

Any help getting these merged in is much appreciated!

[pedromorgan]

Go 1.7 #74 is fixed, its now 1.2 to 1.19

Dropping everything below 1.5 would be handy (no imports)

[harrisonhjones]

@ivy @pedromorgan do y'all still need help with this package? It looks like it hasn't seen some love in a little bit of time. I'd like to help if I can.

[afk11]

@harrisonhjones Happy to help too. I suggest we discuss this on #68 and discuss yet another fork if needs be.