Simplify code at a few places

Remove some duplicate checks,
and unnecessary errors.
Don't set an empty target

... in setupAnnotation. Simply skip the anchoring
cycle when there are no targets.
Don't call scan() in loadAnnotations()

Previously, loadAnnotations() were written such a
way that it made it sure that scan() was run
before trying to anchor any annotations.

After removing this feature, now it's the
responsibility of the application code
to call scan() before loadAnnotions().

(Of course this only matters when the
`dontScan` option was given to the
application, because otherwise, we do
a scanning right away in in constructor.)
Merge pull request #90 from hypothesis/simplify

Simplify code at a few places