add build_date to site, set during build

[jberger]

Before this commit any functionality that depended on knowing the build date (which is possibly overridden by the command line) would need to have the date passed in to it. With this change the site now knows what the build date is and is therefore accessible by anything with access to the site object.

[coveralls]

Coverage increased (+0.02%) to 89.943% when pulling 8bc965cdd5ec5c784a0b7c4364fb842534bb44d5 on jberger:build_date into 3e13a9040721b3306dd734d1189cb538c837ef03 on preaction:master.