Add spike counts to pre-commit tests.

[thorstenhater]

• Enhance run_cpp_examples.sh to do some very basic checking of spikes counts
• Add a script to check all releases
• The former must succeed for all commits to master

Here's the expected spike counts as of 0.7

• brunel 6998
• bench 972
• ring 94 (19 pre-v0.7)
• gap_junctions 30