Refactor deployment

This should fix #373 (and #339)

 * Use rbenv
 * Fixes for Centos

I didn't overly test the development deploy.

Wiki documentation would need to be updated slightly.

```
{

"name" : "huginn_production",

"chef_type" : "role",

"json_class" : "Chef::Role",

"description" : "Huginn Production Environment",

"default_attributes" : {
  "mysql": {
    "server_root_password": "password",
    "server_repl_password": "",
    "server_debian_password": ""
  },
    "nginx" : {
       "init_style" : "upstart"
    }
},

"run_list":[
             "recipe[git]",
             "recipe[apt]",
             "recipe[mysql::server]",
             "recipe[nodejs::install_from_binary]",
             "recipe[nginx]",
             "recipe[ruby_build]",
             "recipe[rbenv]",
             "recipe[huginn::ruby]",
             "recipe[huginn::user]",
             "recipe[huginn::production]"
           ]
}
```

The roles now share most of the code for setup, with only the ```huginn::production``` and ```huginn::development``` recipes differing.

You can now pick your own attributes, like ruby version, git repo, etc:

```
default['rbenv']['ruby_version'] = "2.1.2"
default['rbenv']['plugins'] = [{
  'name' => 'rbenv-sudo',
  'git_url' => 'git://github.com/dcarley/rbenv-sudo.git'
}]

default['huginn']['repo'] = "https://github.com/cantino/huginn.git"
default['huginn']['branch'] = "master"
default['huginn']['rails_env'] = "production"
default['huginn']['keep_releases'] = 5
default['huginn']['user'] = "huginn"
default['huginn']['group'] = "huginn"
default['huginn']['deploy_path'] = "/home/#{node['huginn']['user']}"
default['huginn']['env']['invitation_code'] = "try-huginn-secretly"
```

Mail details still probably need refactoring.
Also, @cantino [said on my branch](https://github.com/cantino/huginn/pull/339/files#r13032763) that these should be commented out with a note if they're included in the Gemfile, as not everyone will want/need them for a normal install.