Fix swipe action with target element: #3647

@Jonahss @imurchie (this reviewers of the first merge request that I closed because the new changes are a lot different) 

*  The original scope of this change was adding the ability to specify a destination element when doing a swipe.  Those are the changes you'll see in the server JS.
*  I added a bunch of tests.
*  When I started expanding the test suite for Android, I uncovered some nasty bugs w relative coordinates.  At least one was caused by copy/pasting.  I then discovered similar code (not nec buggy, but using the same patterns) in multiple places in the project.  I consolidated the logic to decrease the likelihood of bugs and ensure that absolute coordinates were all derived in the same way.
*  There's a bunch of spaghetti code.  If you have any suggestions on how I can get rid of it, lmk.
*  There's a change to the sample code that goes along with this.
*  I ran all the Android `touch` tests and they passed, with the exception of 'should work like `longPress` when released after a pause', which also failed for me before the refactoring


You're not being dense :)  This is a tangle of code. ( oooh, that's a
good candidate for a collective noun for code changes. "I reviewed
your tangle.")

It's in PositionHelper.getAbsolutePosition. In the condition where the
coordinate is 0, 0.5 gets used instead.

On 10/31/14, Jonah <notifications@github.com> wrote:
>> @@ -64,8 +66,9 @@ public AndroidCommandResult execute(final AndroidCommand
>> command)
>>        } else { // no element so extract x and y from params
>>          final Object paramX = params.get("x");
>>          final Object paramY = params.get("y");
>> -        double targetX = 0.5;
>> -        double targetY = 0.5;
>> +        double targetX = 0;
>
> Can you point me to the exact place where the defaults are handled? Sorry
> for being dense.
> Can we just leave them undefined? Or does the `Point` class require real
> coordinates.
>
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> https://github.com/appium/appium/pull/3943/files#r19696693