Update formatting of build.xml
Exclude docs directory from version control

The docs directory will be generated by JavaDoc so should not be
included in the repository.
Add JavaDoc target to build configuration

To generate JavaDocs, use:

    $ ant docs

JavaDoc will then generate documentation for the Stack Answers source
and place it in the docs directory.
Add code documentation to NetworkActivity.java
Assert that tab class must be Fragment subclass
Add Stack Exchange link to README