First working version of the gem.

This commit includes way too much stuff to be listed here but I rather
talk about the goal behind this commit.

First of all, Ecrire can now launch a server by using the following
syntax in irb (given that the gem is installed):

require 'ecrire'; Ecrire.launch!

Ecrire::Application is a direct descendant of Rails::Application and
this is the app that is launched. When Ecrire.launch! is called, the app does a few
things:

- Include all the necessary gems for Ecrire to work
- Register a new railtie in lib/ecrire/themes/railtie.rb
- Configure the application throught lib/ecrire/config/environment.rb
- Launch the server through Rack::Server at port 3000 and host 0.0.0.0
  (localhost)

One thing that is VERY different from a normal application is how
Ecrire::Themes::Railties uses initializers to allow for themes to be
selected.

Ecrire comes with a default theme and controllers and an admin
interface. The admin & controller are the only things that cannot be
customized by the user through different themes.

For everything else, Ecrire looks at the current working directory from
where Ecrire.launch! was called and try to locate database.yml which it
will parse like you'd expect. It then parses secrets.yml and locate the
:theme key. If it's set, Ecrire will load that theme. If not, it will
use the default theme.

A custom theme is defined as follow:

In your working directory, you need to have the following folder
structure:

helpers/{theme_name}/*
assets/{theme_name}/javascripts
assets/{theme_name}/stylesheets
decorators/{theme_name}/*
views/{theme_name}/*
secrets.yml
database.yml

I believe that those 2 yml could probably be moved to a config/ folder
to follow Rails' structure (no need to customize everything)