cast argv to unicode before testing

otherwise the use of `unicode-literals` causes errors
when there are non-ascii characters on argv on Python 2.
Merge pull request #2534 from minrk/unicode-argv

cast argv to unicode before testing
Fix for compiling ttconv in extern directory
Merge pull request #2535 from ianthomas23/extern_directory_fix

Move external libraries to 'extern' directory - correction