Fix location of scripts

[felixdivo]

1. Placing the scripts in a separate module in can.modules.* instead of can.* made them impossible to be called the old way, even with them being present in the can/__init__.py. Thus, the relocation of the scripts to the separate module is reverted.
2. Now, the scripts print the help message when no arguments were given.
3. Adds some tests to ensure that the scripts can always be called.

Fix #392

[codecov]

Codecov Report

Merging #406 into develop will decrease coverage by 3.87%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #406      +/-   ##
===========================================
- Coverage    59.26%   55.38%   -3.88%     
===========================================
  Files           55       54       -1     
  Lines         4328     4407      +79     
===========================================
- Hits          2565     2441     -124     
- Misses        1763     1966     +203