Only set REBAR variable if its not set

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Update tools.mk with version 0.5

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Add appropriate .gitignore lines.
Remove undefined type use

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orddict:orddict/2 is not an exported type, so we simply
type the underlying representation instead.
Manually define xmerl types from records

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xmerl does not define types for its records, so we just
manually define them in yokozuna.hrl.
Ignore application:set_env dialyzer errors

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fix various dialyzer warnings
re-instated vclock sort
Merge pull request #327 from basho/refactor/jra/dialyzer-fixes

Dialyzer fixes (do not merge until riak_core 540 is merged)
Update tools.mk to 0.5.2 to fix a couple bugs
Merge pull request #325 from basho/feature/tools.mk-update-0

tools.mk update and dialyzer fixes