Add support to mssql and pdo_dblib

# Uncompleted pull request

## The problem
Both extensions depends on freetds is installed.

I'm planning to add a line to ensure that on both extension definitions at https://github.com/blackjid/puppet-php/blob/master/manifests/extension/mssql.pp#L35

```puppet
php_extension { $name:
    provider         => php_source,

    extension        => $extension,

    homebrew_path    => $boxen::config::homebrewdir,
    phpenv_root      => $php::config::root,
    php_version      => $php,

    configure_params => $configure_params,
    require          => Package['freetds'],
  }
```

For that I need to run first something like this
```puppet
package { 'freetds':
    ensure => '0.91',
  }
```
but as both extension need it I will get a Duplicate declaration error. 

For that I was thinking to add a new class like this.
```puppet
class freetds {
  package { 'freetds':
    ensure => '0.91',
  }
}
```
and include it in both extensions

```puppet
include freetds
```

## The question

Where can I put this class? In a `freetds.pp` file? in what folder?