v0.16.0 Release
7fe73e9 fix(span): make child span clock relative to root span (#628)
ad6c0d4 feat(span): make tracer available on span (#629)
5b8165e chore(deps): bump lodash from 4.17.11 to 4.17.13 (#619)
440c304 chore(deps): bump lodash in /packages/opencensus-instrumentation-http (#624)
919cdea chore(deps): bump lodash in /packages/opencensus-exporter-zpages (#627)
0f2684a chore(deps): bump lodash in /packages/opencensus-propagation-stackdriver (#621)
668e49b chore(deps): bump lodash in /packages/opencensus-nodejs (#620)
7a88580 chore(deps): bump lodash in /packages/opencensus-exporter-zipkin (#626)
96afa18 chore(deps): bump lodash in /packages/opencensus-exporter-stackdriver (#625)
8647f96 chore(deps): bump lodash in /packages/opencensus-instrumentation-https (#623)
b6ebcfa chore(deps): bump lodash in /packages/opencensus-propagation-b3 (#622)
97661db chore(deps): bump lodash in /packages/opencensus-core (#618)
a3542a5 chore(deps): bump lodash.template from 4.4.0 to 4.5.0 (#617)