except: use descriptive variable argument

Allow message in system_except (through use of gcc extension ##args,
which works on clang aswell).
common: show_message: fix output for exceptions without message
application: message takes one param
Add slap

Initial version, probably lacking some features, which will be added as
it starts getting used.
optional: add typedef value_type

Fix typo.
option: formatting

Remove template default parameter.
slap: update

attribute: add to and get template functions

entry: add value convenience function
entry: add to and get template functions with specialization for
optional<>

NOTE: since partial function specialization is not allowed, we create
_M_to struct and call its func from to. (Don't need tod do this for get,
as it is done through overloading.)
convert: add specialization for QString
slap: entry must be const