Add a class SlapConfigParser which overrite ConfigParser.write method to fix parse problem when
configuration like:
[section]
foo += bar

is included in buildout file. softwaretype recipe will generate
buildout file with foo + = bar because ConfigParser doesn't reconize +=
delimiter and read key as "foo +", value as "bar".
Then ConfigParser.write method generate

[section]
foo + = bar
...

This is invalid with buildout version 2

/reviewed-on !100