python-pip: add preinst and postrm scripts

python-pip: add preinst and postrm scripts

Thanks to Xuefer Tinys <xuefer@gmail.com> for reporting this.

Case is:
  opkg install python-setuptools - works fine
  opkg install python-pip - reports conflicts for
                            files easy_install.pth & site.py

So the solution is to add some preinst script that backs up those files
on 'opkg install python-pip' and moves them back on 'opkg remove python-pip'.

Signed-off-by: Alexandru Ardelean <ardeleanalex@gmail.com>