Add promise support for sessionStore functions

[brandonmanke]

Potential solution to #607.

I can add some more tests to improve coverage if necessary.

[brandonmanke]

@dougwilson I can add the should error without callback test for .reload() if that is ok? Also how would you recommend explicitly testing for errors with and without callbacks? Since the tests themselves will be almost identical.

[dougwilson]

Hey, so I added a couple more conditions on Monday to further bump the coverage to help out 👍Feel free to add some new ones as well; I just pushed up without the rebase on Monday just in case you were working on some as well.

I took a look at the coveralls config and it's set only to care about line coverage. So I wouldn't really worry about the reject branches to get it landed. If you want to add them anyway as an exercise, I would just copy-paste the tests at this time.

[brandonmanke]

How would you recommend testing https://coveralls.io/builds/21919265/source?filename=index.js#L385?

[bjohansebas]

closes in favor of #737