Check for 25mb of free disk space before install

Function will check the free space of / (root)

If the free space is less than 25MB it will give directions
  on how to expand the disk.

Fixes #163

automated install/basic-install.sh

@@ -115,6 +115,20 @@ welcomeDialogs() {
 	In the next section, you can choose to use your current network settings (DHCP) or to manually edit them." $r $c
 }
 
+
+verifyFreeDiskSpace() {
+    # 25MB is the minimum space needed (20MB install + 5MB one day of logs.)
+    requiredFreeBytes=25600
+
+    existingFreeBytes=`df -lkP / | awk '{print $4}' | tail -1`
+
+    if [[ $existingFreeBytes -lt $requiredFreeBytes ]]; then
+        whiptail --msgbox --backtitle "Insufficient Disk Space" --title "Insufficient Disk Space" "\nYour system appears to be low on disk space. pi-hole recomends a minimum of $requiredFreeBytes Bytes.\nYou only have $existingFreeBytes Free.\n\nIf this is a new install you may need to expand your disk.\n\nTry running:\n    'sudo raspi-config'\nChoose the 'expand file system option'\n\nAfter rebooting, run this installation again.\n\ncurl -L install.pi-hole.net | bash\n" $r $c
+        exit 1
+    fi
+}
+
+
 chooseInterface() {
 	# Turn the available interfaces into an array so it can be used with a whiptail dialog
 	interfacesArray=()
@@ -557,6 +571,9 @@ The install log is in /etc/pihole." $r $c
 $SUDO mkdir -p /etc/pihole/
 welcomeDialogs
 
+# Verify there is enough disk space for the install
+verifyFreeDiskSpace
+
 # Just back up the original Pi-hole right away since it won't take long and it gets it out of the way
 backupLegacyPihole
 # Find interfaces and let the user choose one