Add a build script to build a stand alone phpctags

I made the following changes to make a build scripts to let people can make s stand  alone phpctags.

1. Rename phpctags to bootstrap.php
2. Add a wrapper script phpctags.sh to wrap bootstrap.php
3. Add empir and build.sh to build phar file

Just change folder to ./build and execute ./build.sh to build phpctags. The final executable phpctags will under ./bin/phpctags.

User can move it to /usr/bin or /usr/local/bin or anywhere they  want.