Tell getopt to parse POSIX compatible.

By setting the environmental variable POSIXLY_CORRECT, getopt behaves
strictly POSIX-compatible.  This fixes the incorrect getopt parsing
style that breaks git-flow in several flavours of Linux.

Many thanks to Thiana for figuring this one out.

This should fix issues #28 and #29.

git-flow

@@ -67,6 +67,11 @@ main() {
 	# load common functionality
 	. "$GITFLOW_DIR/gitflow-common"
 
+	# This environmental variable fixes non-POSIX getopt style argument
+	# parsing, effectively breaking git-flow subcommand parsing on several
+	# Linux platforms.
+	export POSIXLY_CORRECT=1
+
 	# use the shFlags project to parse the command line arguments
 	. "$GITFLOW_DIR/gitflow-shFlags"
 	FLAGS_PARENT="git flow"