reserve sub directories

From coffeeDir, the sub directories of .coffee files reserve.

add preserveSubDirectory option.

### Example

* src/main/coffee/
   * a.coffee
   * sub/
      * b.coffee
      * sub2/
         * c.coffee

#### 1. preserveSubDirectory == false
~~~
<configuration>
   <coffeeDir>src/main/coffee</coffeeDir>
   <coffeeOutputDirectory>src/main/webapp/javascript</coffeeOutputDirectory>
</configuration>
~~~
* src/main/webapp/javascript/
   * a.js
   * b.js
   * c.js

#### 2. preserveSubDirectory == true
~~~
<configuration>
   <coffeeDir>src/main/coffee</coffeeDir>
   <coffeeOutputDirectory>src/main/webapp/javascript</coffeeOutputDirectory>
   <preserveSubDirectory>true</preserveSubDirectory>
</configuration>
~~~
* src/main/webapp/javascript/
   * a.js
   * sub/
      * b.js
      * sub2/
         * c.js