Feature/apt mirror support | MODULES-1243

Hello, 

I have added the support for apt-mirror configuration. Please review it. Here is the link to the ticket created for this one - https://tickets.puppetlabs.com/browse/MODULES-1243.

Class apt::mirror: To install and configure apt-mirror. This class will configure apt-mirror global/base configuration like different directory paths, number of threads, etc. 

Class apt::mirror::source: To configure individual apt-mirror sources with appropriate options.

Thanks,
Harish
Adhere to the documentation style we're using in the rest of the module please: https://github.com/puppetlabs/puppetlabs-apt/blob/master/manifests/backports.pp