Changed execute_adb_command to explicitly use adb as a prefix and improved README.md

[edgan]

Changed execute_adb_command to explicitly use adb as a prefix, and to validate if the adb executable is available.

See:
#3

[minhalvp]

Your first 3 commits makes sense and I would like to merge.
#4 Adding adb as a prefix would change nothing as it is removed again in execute_adb_command in adbdevicemanager.py. I think I need to change the function names to make it more obvious that it is specifically an adb shell command that is being executed by the function

[minhalvp]

I merged the first 3 commits. I provided a response about the 4th commit in #3