support user specified rsync excludes.

This commit allows to specify subfolders that should be excluded from syncing via synced_folder options:
e.g.:
config.vm.synced_folder '.', '/vagrant', :rsync_excludes => ['chef/tmp', 'tmp']
Release Elastic IP before destroying
Fix Vagrant 1.4 compatibility and support multiple SSH keys

Vagrant 1.4.0 added support for multiple SSH keys (mitchellh/vagrant#907)
and changed `private_key_path` to an Array.

This commit ensures we always handle it as array to keep compatible with
both old and new Vagrant versions. Also pass all of the specified keys
to rsync/ssh command.

Fixes #171
Add ProvisionerCleanup to the destroy action middleware stack

Call cleanup method of the provisioners (Vagrant >= 1.3.0).

Fixes #170
Fix #174
Merge pull request #164 from ctso/master

Elastic IP is not released when instance is destroyed
Merge pull request #172 from tmatilai/multiple-ssh-keys

Fix Vagrant 1.4 compatibility and support multiple SSH keys
Merge pull request #173 from tmatilai/provisioner-cleanup

Add ProvisionerCleanup to the destroy action middleware stack
Merge pull request #175 from maljub01/user_switching

Fix #174
use latest vagrant
Merge branch 'rsync_excludes' of github.com:databus23/vagrant-aws into databus23-rsync_excludes

Conflicts:
	lib/vagrant-aws/action/sync_folders.rb
bump fog.io to 1.18.0
release 0.4.1
set version back to dev