Eclipse build changes

If you look at the wiki instructions for <a href="https://github.com/k9mail/k-9/wiki/BuildingK9#building-with-eclipse">building K9 with Eclipse</a>, you will see a long, tedious 40 step process.  That process can be greatly simplified as a result of the changes in this pull request.  Basically, steps 2-36 become:

* File->Import->Android->Existing Android Code Into Workspace->Next
* Browse to the K9 working directory (all the necessary projects will be found and selected for importing)
* Click "Finish"

Building can then proceed and everything will be built in the correct order.

I have been hesitant to suggest these changes previously because there will likely be some short-term inconveniences for those who already have Eclipse set up with K9.  In particular, this removes the top .classpath file from revision control so that, upon checkout, the file will be removed from the working directory of existing setups and building will fail.  So, while the file's removal helps simplify new setups, it complicates existing setups.

Nonetheless, I now think this is the time to make this change because with the recent release of Android SDK Rev. 22, the existing .classpath file results in <a href="http://stackoverflow.com/questions/16596969/libraries-do-not-get-added-to-apk-anymore-after-upgrade-to-adt-22">an APK file that will not run</a>.  So changes are needed one way or another.

Existing setups can subsequently restore a corrected .classpath file with:
git checkout 53fcdd1 .classpath
git rm --cached .classpath

Unless there are objections or concerns, I'll plan to merge this in the next week or so, and then I'll update the wiki accordingly.