Add timezone setting

[UltCombo]

Currently, the generated files' timestamps' timezone is dependent on the OS.

This a problem when team members in different timezones build and deploy the site -- that is, Harmonic will generate timestamps with different timezones when building the site in different computers.

Hence, I propose a new timezone setting in the harmonic.json, so that different environments will consistently generate timestamps in the chosen timezone. I'd suggest defaulting it to GMT (UTC).

I haven't thought about the details/implementation yet, but what do you guys think?

//cc @jaydson

[jaydson]

timezone on the harmonic.json can be a good approach, indeed.