Uw report multiple datapoints

Added the possibility to provide a list of data points in a subscription. The call to `get_value/2` is done once for all data points. After that, they are sent individually to the reporter instance. Thus, from the reporter instance's point of view, it looks exactly the same as before.

Example:
```erlang
2> exometer_report:add_reporter(exometer_report_tty,[{type_map,[{[a,'_'],gauge}]}]).
ok
23:36:37.993 [info] exometer_report_tty([{type_map,[{[a,'_'],gauge}]}]): Starting
3> exometer:new([a],histogram).
ok
4> exometer_report:subscribe(exometer_report_tty,[a],[min,mean,max],10000).
ok
5> exometer:update([a],100).
ok
6> exometer:update([a],200).
ok
exometer_report_tty: a_min 1393367926:100 (gauge)
exometer_report_tty: a_mean 1393367926:100 (gauge)
exometer_report_tty: a_max 1393367926:100 (gauge)
```