Check for unsent/unexported data before creating zip files

* Prevent the creation of empty directories if no data is ready to be
sent/exported.

Replay:
https://github.com/akvo/akvo-flow/commit/cb5f863f2dbdc8d80f99c758c065ea072f3f67b0
Use boolean data type to replace "true" and "false" Strings

Convert the String property into boolean, returning true if the value
matches "true", false otherwise.

Replay:
https://github.com/akvo/akvo-flow/commit/0343e515aa3d94540ce958c87fe3ff6a4bdda4c6
Issue #2 - Implement deletion of files on the device

* Split up data deletion options into 'collected data only' and
'everything'.
* Move data deletion into a background thread (AsyncTask).
* Notify user of data deletion success/failure.
* Add helper methods to Database adapter in order to get the amount of
unsent/unexported data

Replay: https://github.com/akvo/akvo-flow/commit/5c5c0447d165a3736a20b709506694cc5758dbb4
Issue #2 - Move ClearDataAsyncTask to a separate file

* Move this task to a sepatate public class.
* Use a WeakReference to the Context to allow the calling Activity
being GC if it gets finished before the AsyncTask's completion.
Issue #2 - Rephrase data deletion option for a clearer approach
Issue #2 - Rename keepSurveys flag to responsesOnly
Issue #12 - Remove the "Submit Data Only" option

* Force all the data to be sent together (Text Data + Images)
Merge branch 'develop' into feature/deletedata
Issue #14 - Create a helper class to wrap formZip() response

* Define ZipFileData to hold zip file's metadata.
* Use interfaces for declarations. Classes for implementations.
Issue #14 - Split zip creation functionality

* Separate zip creation from data upload
* Use interfaces for declarations, classes for implementations
* Remove unused code
* Refactor long methods
Issue 14 - Handle Amazon S3 upload failure
Issue #14 - Add html to javadoc
Merge branch 'develop' into feature/deletedata
Issue #16 - Apply new Code Style to java files
Issue #16 - Format java files to with a consistent code style

* Remove empty lines before field declarations.
* Remove empty lines before function closure.
* Format comments that got wrapped with the new line length.
* Add empty line before class closure (most files already had this style).
Add code style guidelines to REAMDE
Issue #16 - Add Eclipse formatter
Issue #16 - Format xml files with defined code style
Merge branch 'feature/codestyle' into feature/deletedata
Merge branch 'feature/deletedata' into develop