Review and merge upstream pull requests

[ivy]

The following pull requests need to be reviewed and merged:

• Export NetDialTimeout to public (Export NetDialTimeout to public go-gomail/gomail#100)
• Added Check For Existing Files And Attachments (Added Check For Existing Files And Attachments go-gomail/gomail#97)
• Add syntax highlighting to Go example (Add syntax highlighting to Go example go-gomail/gomail#92)
• ~~~fix mail attachment name garbled (fix mail attachment name garbled go-gomail/gomail#83)~~~ - This one needs more attention to be sure charset encoding is being handled correctly.
• ~~~Break out SMTP Client Reset Method (Break out SMTP Client Reset Method go-gomail/gomail#79)~~~ - Moved to Review and merge API breaking changes #8
• Accept io.Reader for Attach and Embed (Accept io.Reader for Attach and Embed go-gomail/gomail#78)
• ~~~Added Go 1.7 to .travis.yml (Add 1.7 testing. go-gomail/gomail#73 Added Go 1.7 to .travis.yml go-gomail/gomail#74)~~~ - Fixed in 92faf95
• Allow timeout in connections, retries to be configurable (Allow timeout in connections, retries to be configurable go-gomail/gomail#70)
• ~~~Add support for attaching/embedding files for URLs (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.