exp/locale/collate: Added functionality to parse and process LDML files

for both locale-specific exemplar characters and tailorings to
the collation table.
Some specifices:
- Moved stringSet to the beginning of the file and added some functionality
  to parse command line files.
- openReader now modifies the input URL for localFiles to guarantee that
  any http source listed in the generated file is indeed this source.
- Note that the implementation of the Tailoring API used by maketables.go
  is not yet checked in. So for now adding tailorings are simply no-ops.
- The generated file of exemplar characters will be used somewhere else.
  Here is a snippet of how the body of the generated file looks like:

type exemplarType int
const (
        exCharacters exemplarType = iota
        exContractions
        exPunctuation
        exAuxiliary
        exCurrency
        exIndex
        exN
)

var exemplarCharacters = map[string][exN]string{
        "af": [exN]string{
                0: "a á â b c d e é è ê ë f g h i î ï j k l m n o ô ö p q r s t u û v w x y z",
                3: "á à â ä ã æ ç é è ê ë í ì î ï ó ò ô ö ú ù û ü ý",
                4: "a b c d e f g h i j k l m n o p q r s t u v w x y z",
        },
        ...
}

R=r
CC=golang-dev
https://golang.org/cl/6501070

src/pkg/exp/locale/collate/tables.go

 // Generated by running
-//  maketables --ducet=http://unicode.org/Public/UCA/6.0.0/allkeys.txt
+//  maketables -root=file:///Users/mpvl/Projects/go/hg/gopub/src/pkg/exp/locale/collate/allkeys.txt -cldr=file:///Users/mpvl/Projects/go/hg/gopub/src/pkg/exp/locale/collate/core.zip
 // DO NOT EDIT
 // TODO: implement more compact representation for sparse blocks.