Cleaned up code, improved README and fixed issues with serial port

[rzbrk]

Fixes issues #5 and #13

For issue #13 I made some major changes to the code.

• I used serial.tools.list_ports.comports() in the function getMiniSerial() to retrieve a list of serial ports from the OS. The serial port for the Calliope Mini is identified by "Calliope mini - CDC". This way, the port of the Calliope Mini can be reliable determined.
• The pySerial instance ser is no longer a global variable. The serialConnect() routine will return the instance in the case, the connection is successful.
• The function programmMini() no longer contains the connection to the Calliope Mini. Instead, the connection is triggered from main() via calling the function connectSerial().
• I found out, that the serial port of the Calliope Mini shows up very early after programming. On can also connect very soon. But reading/writing too soon will cause an I/O error. The best work around was to wait 10 seconds after the serial port could be found in the system before connecting and making any read/write operations.

I tested the code on my Raspbery Pi. The scripts executes as expected with and without Calliope Mini.

[rzbrk]

I just saw, that the dependency to package glob is no longer needed, because the script now uses serial.tools.list_ports.comports() to retrieve a list of serial ports from the OS. We can delete import glob. I added a dedicated commit.