Added exometer_function:eval_exprs/2 and expr evaluation in metrics

An abstract expression language, similar to Abstract Erlang, but
slightly more compact, is introduced for complex handling of function
call results. The language is described in the docs for
exometer_function:eval_exprs/2.
Merge branch 'master' of github.com:Feuerlabs/exometer into uw-function-eval
Merge pull request #58 from Feuerlabs/uw-function-eval

Uw function eval