(#272) Add parameter sslproxyengine to vhost.pp

In order to use mod_proxy with remote servers using the SSL/TLS protocol it is
needed to activate the SSL/TLS Protocol Engine for proxy. Setting this
parameter to true adds the directive `SSLProxyEngine on` to the virtual host.
Remove sendfile comment

The comment in the conditional block for sendfile directive inclusion
caused the plain text directive name to not be included in the script.
(#278) Ports file attempted to be created before Apache installed

Ports file creation should depend on Apache being installed.
Add KeepAlive parameter

KeepAlive parameter was added to init.pp (by default is equal apache::params::keepalive), params.pp (by default is 'Off') and template httpd.conf.erb
Desription was added to README.md
Test against facter 1.6
Merge pull request #282 from hunner/update_facter_travis

Test against facter 1.6
Merge pull request #280 from thegriglat/master

Add KeepAlive parameter
Merge pull request #279 from puphpet/bug/master/ports-requires-apache

(#278) Ports file attempted to be created before Apache installed
Merge pull request #274 from shanethehat/remove-sendfile-comment

Remove sendfile comment
Merge pull request #273 from mattthias/feature/master/add_sslproxyengine_to_vhost

(#272) Add parameter sslproxyengine to vhost.pp