Make it easier to get started with Eclipse.

I'm proposing a few changes to make life of Eclipse users a little easier. This pull request makes `mvn eclipse:eclipse` generate additional eclipse configuration
files so that Eclipse:
 - uses Java 1.6 compliance level,
 - truncates lines after 140 chars,
 - uses 4 spaces for indentation,
 - automatically adds a license header when creating a new class file,
 - organizes imports the same way as Intellij Idea (which makes sense I guess
   since most of the code bas has been written with Intellij, this will prevent
   from having large diffs due to the fact that the order of imports has
   changed).

This should make getting started coding on Elasticsearch as easy as:
 - `mvn eclipse:eclipse`
 - `File > Import > Existing project into workspace` from Eclipse
 - code!

What do you think?