Use relative paths in install script template

tools/niminst/install.tmpl

@@ -3,7 +3,35 @@
 #  result = "#! /bin/sh\n# Generated by niminst\n"
 #  var proj = c.name.toLower
 
-set -e
+## Current directory you start script from
+BASE_DIR=$(pwd)
+
+## The following one-liner takes directory path which contains install script.
+## `command -v -- "$0"` takes path if script sourced from interactive shell
+## `dirname` returns relative directory path to install script
+## `cd -P` dive into directory to use `pwd`
+## `pwd -P` prints full path to install script directory path
+## -P option allows to use symlinks in path
+## Good explanation can be found here:
+## http://stackoverflow.com/questions/29832037/how-to-get-script-directory-in-posix-sh
+NIM_DIR=$(cd -P -- "$(dirname -- "$(command -v -- "$0")")" && pwd -P)
+
+go_back() {
+  cd $BASE_DIR
+}
+
+## Go to base dir on exit
+trap go_back EXIT
+
+install_error() {
+  echo "Nim installation failed!"
+  exit 1
+}
+
+## Exit if any command failed
+trap install_error ERR ## `set -e` alternative
+
+cd $NIM_DIR
 
 if [ $# -eq 1 ] ; then
 # if c.cat[fcUnixBin].len > 0: