Breakup examples to their own files in examples/user/user.java

[thinkingserious]

File user.java has 422 lines of code (exceeds 250 allowed). Consider refactoring.

Break up these examples into their own files, named after the action being shown.

https://codeclimate.com/github/sendgrid/sendgrid-java/examples/user/user.java#issue_59f6aee388b681000100005d

[thepriefy]

I'll take this :)

[thepriefy]

I wanna do this one... @pushkyn :(

[mbernier]

@thepriefy it looks like @rivenhk had already done it and @pushkyn referenced this issue from Rivenhk's PR.

[mbernier]

Once I get #370 merged, we will see if this issue still needs to be done.