fix slice and counts, and add modifiers

[nicolasfranck]

when computing the appropriate start en limit values for solr, when doing a slice, one should remain within the boundaries of a previous slice; the same for count.

[nicolasfranck]

Remark: the combination of a slice and select can be tricky

a select->slice is not the same as slice->select.

Consider this array: 0,1,2,3,4,5,6,7,8,9

• slice(2,2)->slice(1)->select( odd => 1 )

  [2,3] => [3] => [3]

• select( odd => 1 )->slice(2,2)->slice(1)

  [1,3,5,7,9] => [5,7] => [7]

This is in fact what would happen if the Solr package did not implements these methods,
and the default slice and select would happen (using the generator).

I had a look at how Rails does this, and it's not different there:

Model.limit(2).offset(1).where( "foo" => "bar)

creates the same query as

Model.where( "foo" => "bar).limit(2).offset(1)

Not what todo in such situations. In normal cases select->slice is used?

[nicolasfranck]

What should work already:

slice->slice ..
select->select
select->slice

This..

slice->select

should be fixed by adding an extra check in the "around select" modifier:
only if start is 0 and total not provided (read: no slice), should one generate a new Searcher-object . See latest commit.