Improve report interface

[bernardolins]

This pull request implements changes that make the Alchemetrics report interface more flexible. Changes include:

• The Alchemetrics.report/2 function no longer accepts datapoints as an argument. Instead, it will always report all data points. Also, it is no longer necessary to provide an atom as a metric name, using metadata as the metric identifier. However, for cases where metadata is not needed, it is still possible to create a metric where the name is just an atom.

• To cover cases where it is only necessary to count the occurrence of an event in a given time interval, the functions Alchementrics.increment/1 and Alchementrics.increment_by/2 have been created. Both behave like the Alchemetrics.report/2 in the way the metric is identified.

• Due to the need to remove the metric name, CustomReporters no longer receive this attribute in the report/4 function.

• The option :instrument_beam was created, which allows the user to choose whether or not he wants to instrument data from the Erlang VM.

[coveralls]

Changes Unknown when pulling cbc92f8adb1b875a8321cde764a12a875a34da16 on bernardolins:master into on globocom:master.

[coveralls]

Changes Unknown when pulling 4c8f0f32b5af1a490dfd3434692fb325e4037117 on bernardolins:master into on globocom:master.