Support UTC timezone

[gildorwang]

Sometimes I need to deal with UTC dates so I updated a little bit:

1. Add timezone configuration. Accepted values: null or UTC
2. Add timezone parameter for angular date filter to show date in UTC timezone
3. Change date format to ISO 8601 format so that date string will be parsed as UTC timezone (only when the timezone configuration is set to UTC)

[gildorwang]

This should address #74.

[lukebyrne]

Whats the status of this merge request? I need the same functionality. @adamalbrecht I see that you are doing a rewrite, does this mean all PR's are being ignored for the time being?

[adamalbrecht]

@lukebyrne Not necessarily ignoring - I'm just evaluating each one individually to see whether it's worth it at the moment.

In the rewrite, my intention is to make it more focused and modular so that dealing with issues like time zones and date formats is done outside of the library. Otherwise there are just too many edge cases to worry about.

[lukebyrne]

Cheers, I ended up just using the angularJS inbuilt stuff you had to get the UTC offset I needed for the time being. Thanks for an awesome plugin.

[gildorwang]

I have coded the support for UTC timezone in my fork (with tests):

https://github.com/gildorwang/ngQuickDate

Anyone who needs this feature can have a try!