Add option to show browser's local time

[NikitaIvanovV]

This PR adds an option to show browser's local time when viewing history, commit, latest_changes and clicking When was this page last modified? button.

I used the client-side approach: I pass time in datetime attribute of <time> tag and then JS creates a Date object, which will be in borwser's local time. To support strftime (used for formatting Time in latest_changes and history) on the client, I had to include Datejs library. I hope adding another library for such a minor thing is not a big problem?

[NikitaIvanovV]

Oh no, I forgot to add tests again. Will do tomorrow!

[dometto]

@vichyavin this looks good to me. Perhaps you could change the documentation strings a little:

Show browser's local time instead of server's.

...is a bit unclear. If I understand the point of this PR correctly, it would perhaps be better phrased by:

`Use the browser's local timezone instead of the server's for displaying dates."

@bartkamphorst any thoughts?

[dometto]

@vichyavin it also looks like one of the tests is failing on JRuby:

https://travis-ci.org/github/gollum/gollum/jobs/754610273

Could you investigate? Thanks!

[NikitaIvanovV]

@dometto Done!

[dometto]

Thanks @vichyavin. One more suggestion for refactoring in the comments. Sorry for not mentioning it earlier.

[dometto]

Again, thanks for the contribution and patience, @NikitaIvanovV!