Simplify the usage of the DA API:

- add a convience method to return the number of installed DPs;
- add a convience method to install a DP that closes the input stream after
  the DA is finished.



git-svn-id: https://svn.apache.org/repos/asf/felix/trunk@1587474 13f79535-47bb-0310-9956-ffa450edef68