Better CircleCI setup

[MartinHelmut]

Current CircleCI setup is very basic (just made it work). Needs more love, at least the following:

• Dedicated steps for build and test (Better CircleCI setup with dedicated jobs #47)
• Only run on code change
• Different compilers
• Cache dependencies
• Different OS'

For "Only run on code change", this could be done via a checksum, example script for this is e.g.:

#!/bin/bash
last_checksum=3ff06124af86b3a8ee43325f93769da9

checksum=`find src -type f -exec md5 {} \; | sort -k 2 | md5`

if [ "$last_checksum" = "$checksum" ]
then
    echo "nothing"
else
    echo "change"
fi