Add config file handling

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Document configuration file

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Add keyring options

Add --keyring and --no-keyring to the command line options and keyring to
section [general] in the configuration file.

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
also use logging module output with --debug
use logger for debug output
use logger for print_error
only log INFO to console, DEBUG to file
different logger for backends
prepend ERROR also without --debug
start logging before option parsing

Additionally log some version informations as INFO,
rather than printing it.
windows: use libdiscid drive detection if available

libdiscid >= 0.6.0 can detect which drive letter
is the first cd drive. So we want to use that,
rather than just using D: as a default.
Merge branch 'logging'

see pull request #87
Merge branch 'w32_drive'

see pull request #86

Conflicts:
	isrcsubmit.py
Get rid of fill_config_with_default_values

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Check if selected backend is valid

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Enable keyring per default

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Merge branch 'config'

see pull request 84
add missing comma
remove print_error2() call
more detail in config man page

Show usage for boolean.
The user setting will probably be used by many users.