Modify PATH passed to cabal-install or cabal-dev

Cabal-install or cabal-dev won't have "$INSTALL_DIR/bin" in their path
by default. When installing more then one package in to the same sandbox
it is sometimes useful that the tools installed in previously are
available to the package being currently installed.

Introduced options:

* -p, --modify-path (enabled by default)
* -P, --preserve-path (disabled by default)
* -d, --create-bin-dir (disabled by default)
Documenting script for installing packages in a sandbox