Add @Metric and deprecate @InjectMetric

New annotation `@Metric` will inject a metric, register the metric, or replace the metric in that field. `@InjectMetric` is also deprecated.

```java
    @Metric
    Histogram histogram;

    @Metric
    Histogram uniformHistogram = new Histogram(new UniformReservoir());
```

In this example, field `histogram` will have an instance of Histogram injected (which defaults to using a BiasedReservoir), while field `unbiasedHistogram` will have its instance of Histogram (using a UniformReservoir) registered with the MetricRegistry (if multiple instances of this bean exist, subsequent beans will have the field `unbiasedHistogram` overwritten with the instance of Histogram which was previously registered with the MetricRegistry)

This resolves #38.