Add install_composer.sh script

Changelog

@@ -1,3 +1,5 @@
+2019-08-15 DariusIII
+	* Chg: Add install_composer shell script to install latest version of composer
 2019-08-14 DariusIII
 	* Chg: Add and start updating nntmux:resetpp command
 2019-08-13 DariusIII

install_composer.sh

@@ -0,0 +1,18 @@
+#!/bin/sh
+
+EXPECTED_SIGNATURE="$(wget -q -O - https://composer.github.io/installer.sig)"
+php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');"
+ACTUAL_SIGNATURE="$(php -r "echo hash_file('sha384', 'composer-setup.php');")"
+
+if [ "$EXPECTED_SIGNATURE" != "$ACTUAL_SIGNATURE" ]
+then
+    >&2 echo 'ERROR: Invalid installer signature'
+    rm composer-setup.php
+    exit 1
+fi
+
+php composer-setup.php --quiet
+RESULT=$?
+rm composer-setup.php
+mv composer.phar /usr/local/bin/composer
+exit $RESULT