getting indiv props instead of reading all props (Netflix#444)

Secured applications may not have access to all system properties on
a server. Making this change to prevent reading all the system properties
and just read the properties that are required for creating the
DefaultMonitorRegistry instance.

Fixes Netflix#443.

v.0.12.20

Adds `DynamicGauge`

fix name of publish env variable (Netflix#440)

It was `BINTRAY_PUBLISH` in the travis config but the
script is checking `GRADLE_PUBLISH`.

v0.12.18

Dependency updates, plus counters for errors getting values from
monitors.

variance is imprecise when size is small (Netflix#418)

In my opinion the current variance (and by consequence
the standard deviation) computation are imprecise, when
curSize is not high (when curSize < 10 for example):

```java
    variance = (sumSquares / curSize) - (mean * mean);
```

The computation I suggest in this pull request does not hurt
and is precise:

```java
    if (curSize == 1) {
      variance = 0d;
    } else {
      variance = (sumSquares - ((double) total * total / curSize)) / (curSize - 1);
    }
```

For reference:
http://web.archive.org/web/20050512031826/http://helios.bto.ed.ac.uk/bto/statistics/tress3.html

v0.12.15

* Updated AWS SDK dependencies

v0.12.14

* Atlas validation rules for groups

v0.12.13

* Reduce garbage generated in `ValidCharacters#toValidCharset`
* MonitorTags annotation support from parent classes
* Style fixes

reduce thread contention for expiring cache