Modernize views, view controllers, and other classes.

* Use properties where possible, and avoid declaring ivars
* Replace most nibs by using Storyboards
* Replace FlexiCell and supporting classes with one class that
uses Auto Layout
* Make cells for displaying tasks using Auto Layout and ReactiveCocoa
* Reorganize project structure to group classes by role
* Update README.md with instructions for init'ing the ReactiveCocoa
submodule and adding a DropboxApiKey.h file
* Add ReactiveCocoa v1.9.6, as a submodule, and use it to simplify use of
DropboxFileDownloader & Uploader

Also add a view for filtering tasks.
Merge pull request #171 from brendonjustin/feature/modernize

Modernize views, view controllers, and other classes.
Update copyright year, closes #166
Ignore blank lines from todo.txt file
Closes #168, closes #170
Match Add Task hint text to Android version
Remove release statements
Improve branding
Remove sync button. Use a UIRefreshControl instead.

Also remove app delegate class methods.

TODO: Refactor out this functionality from the app delegate entirely.
Better organize methods in TasksViewController
Use different layout constants for TaskCell on iOS <7 than on 7+.
Merge pull request #180 from brendonjustin/feature/pullToSync

Use a pull-to-refresh control instead of a button for syncing, closes #151
Merge pull request #181 from brendonjustin/feature/taskCellLayoutOn7

Use different layout constants for TaskCell on iOS <7 than on 7+. Closes #178
Add accessibility labels to Task list, Task addition

Added accessibility label to the task list.  The label will allow
Voice Over users to hear their tasks spoken to them, and also provide a
hook for testing frameworks.

Labels are composed of the priority, task, and creation date, the full
version being 'Priority B, Complete Tax Return, Created Yesterday".

Added the accessibility label "Task Details" to the text field in the
"Add Task" view.

Closes #183, closes #108
Support adding multiple tasks at once.

Tasks are delimited by newlines during entry.
Update existing unit tests to use the new method to add tasks.
Merge pull request #179 from brendonjustin/feature/multipleTaskEntry

Support adding multiple tasks at once. Closes #175
Update hint text and Add Task view title to reflect batch entry
More name-changing from Todo.txt Touch to Todo.txt
Keep the identifier bundle consistent
Version 1.4
Make Log in button green (stopgap solution until we redesign whole screen)