Allow to define custom IP resolver
Update README.md

Add description of custom IP resolver
Merge sevos pull/15 into master

Merge branch 'enable_using_private_ips_from_aws' of
https://github.com/sevos/vagrant-hostmanager into master.

Conflicts:
	lib/vagrant-hostmanager/config.rb
	lib/vagrant-hostmanager/hosts_file.rb
Merge branch 'sevos-enable_using_private_ips_from_aws'