Fixes #1092, Adds a format option to the {{isotime}} variable.

Now using the golang magic date: Mon Jan 2 15:04:05 -0700 MST 2006
One can format the time like:

    {{isotime "2006-01-02"}} == "YYYY-MM-DD"
    {{isotime "060102-15"}}  == "YYMMDD-HH" (24-hour clock)
    {{isotime "060102-3"}}   == "YYMMDD-H"  (12-hour clock)

Using {{isotime}} as a standalone variable doesn't change. It still returns RFC3339 formatted time.
Adds documentation to the website. Explains how to format the time.
Merge branch 'template-timefmt' of github.com:njones/packer into njones-template-timefmt

Conflicts:
	packer/config_template.go
	website/source/docs/templates/configuration-templates.html.markdown
Merge branch 'njones-template-timefmt'
Update CHANGELOG