Use specified duration in Efx preview
Remove parameter 'direction' - replace it with efx's own direction().
Add unit tests for backward direction.

The backward path is not an exact copy of the forward path due to rounding,
but it's good enough for now.
Take effect's direction in account.

Now both EFX::direction() and EFXFixture::direction() are considered.
EFXFixture::direction() is relative its EFX's direction().

That means if both are Backward, it is the same as if they both were
Forward.

Until now, only EFXFixture::direction() was used for the movements.
Remove unused return value
Display fixture positions in the EFX preview.

- probably works with low fixture count (<9)
- only the lowest number for each direction is displayed
Add start offset to EFX.

Start offset allows to start the movement in any point of the path and complements the rotation parameter
for asymmetric patterns.
Merge pull request #117 from janosvitok/efx-improvements

Improvements to EFX module. Added start offset, updated documentation