#4 Add merge code for BrowseResult object

[John Wang]

We will have to do distributed browsing sometime once
an index gets large.

We need the ability to merge an array of BrowseResults
into one.

We would merge document arrays as well as the choices.

basically implement a method:

static BrowseResult merge(BrowseRequest
req,BrowseResult[] results);

Let's put this method in BrowseResult class.

[Albert]

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Implemented: browse hit merging and the simple choice merging.
TODO: choice merging to be moved to the ChoiceCollection
class using the latter's knowledge of what a top choice is;
also, range choice isn't trivial - and isn't done.

[John Wang]

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This is fixed except for merging of dynamic ranges. Lowering the priority.