Add build timestamp to version data

[otoolep]

This was the final missing part of the version. This makes it trivial to be absolutely sure you're working with, say the latest nightly.

Example output:

$ ./influxd version
InfluxDB v0.9.5 (git: master 356871a1b1c2eb2bb682501984b7ef37307c81e7, built 2015-09-25T06:32:21+0000)

> SHOW DIAGNOSTICS FOR 'build'
name: build
-----------
Branch  Build Time                      Commit                                          Version
master  2015-09-25T06:32:21+0000        356871a1b1c2eb2bb682501984b7ef37307c81e7        0.9.5

[otoolep]

@corylanou

[corylanou]

+1