improve trace output for executed commands + drop requirement for --experimental for --trace

[boegel]

Example output with EasyBuild v3.4.0:

== configuring...
  >> running command './configure --prefix=/Users/kehoste/.local/easybuild/software/zlib/1.2.11' (output in /tmp/eb-508__5/easybuild-run_cmd-GQnYWm.log) [started at: 2017-09-15 20:58:04]
== building...
  >> running command 'make -j 4 CFLAGS="-O2 -fPIC"' (output in /tmp/eb-508__5/easybuild-run_cmd-NXCfvW.log) [started at: 2017-09-15 20:58:05]

Example output with this patch included:

== configuring...
  >> running command:
	[started at: 2017-09-15 20:56:14]
	[output logged in /tmp/eb-c13cXN/easybuild-run_cmd-pV_hJa.log]
	./configure --prefix=/Users/kehoste/.local/easybuild/software/zlib/1.2.11
  >> command completed: exit 0, ran in 1s
== building...
  >> running command:
	[started at: 2017-09-15 20:56:15]
	[output logged in /tmp/eb-c13cXN/easybuild-run_cmd-i0AnOq.log]
	make -j 4 CFLAGS="-O2 -fPIC"
  >> command completed: exit 0, ran in 3s

[wpoely86]

this can be done better I hope? Take total_seconds, use it as timestamp input and use the buildin formatting functions?

[boegel]

fixed, but apparently this doesn't work with Python 2.6... :(

AttributeError: 'datetime.timedelta' object has no attribute 'total_seconds'