Hello 

If you want module should auto install at the time of database creation you can make it auto installable by changeing __openerp__.py file like:

# -*- coding: utf-8 -*-
{
    'name': 'Auto install',
    'category': 'Hidden',
    'version': '8.0.1.0',
    'description':
        """
OpenERP auto instal module
=============================
        """,
    'depends': ['web'],
    'auto_install': True,
    'init_xml' : ["static/src/xml/template.xml",],
    'qweb' : ['static/src/xml/base.xml',],
    'js' : ['static/src/js/main.js'],
    'bootstrap': True,
}

If you want to install module on any button event from your custome module you can use below code:

module_obj = pool.get('ir.module.module')

module_list = ['sale','purchase']

module_ids_to_install = module_obj.search(cr, 1, [('name','in',module_list)])

module_obj.update_list(cr, 1, context=None)  // just to update module list to update new module added

module_obj.button_immediate_install(cr, 1, module_ids_to_install, context=context)

Updated for v15:

Inside a Server Action / Automated Action / Scheduled Action:

env['ir.module.module'].search([('name','=','stock')],limit=1).button_immediate_install()

Inside a module:

self.env['ir.module.module'].search([('name','=','stock')],limit=1).button_immediate_install()

in __openerp__.py,  set 'auto_install': True,