Osgi fix imports 

I noticed that version 1.0.6 does not work well with OSGi because it includes all the classes from ftplet-api but it does not export them in the manifests headers. This requires adding the ftplet-api bundle which results in two versions of the same classes on the classpath. This makes custom implementation of Ftpserver interfaces impossible. 

I tried to build my own UserManger and User implementation to authenticate against a DB via JPA. I could not do it because I got linkage errors since my implementation depended on the ftplet-api interfaces which are loaded in OSGi via two separate classloaders: one version from the ftpserver-core bundle (but these are private and not exported) and one from ftplet-api. 
  
~~~
        <dependency>
            <groupId>org.apache.ftpserver</groupId>
            <artifactId>ftpserver-core</artifactId>
            <version>1.0.6</version>
        </dependency>
~~~