Allow priority anywhere in the text

Often I find myself thinking of the priority after I wrote the todo text. Then I have to go back to the start of text and insert the priority.
This modification allows the (X) notation anywhere in the text. If encountered, it will be placed at the start.
Print usage - fixes issue #2
Treat absent due dates as infinitely in the future

The simplest way to achieve my desired effect of sorting tasks without due dates after those which are deadlined.  I don't know if it's just me personally, but I find this makes a lot more sense. :)
Merge pull request #7 from mruwek/fix-help-add

Print usage - fixes issue #2

Sorry about the delay, I was not notified about your pull request. Thanks.
Merge pull request #8 from VPhantom/patch-1

Treat absent due dates as infinitely in the future
Make quotes optional in add subcommand (fixes #3).

Before, only this invocation was allowed:

    todo.sh add "This is a test."

Now you can also enter:

    todo.sh add This is a test.