Export constants for trace.Status codes (census-instrumentation#734)

Release 0.8.0 (census-instrumentation#686)

Updates census-instrumentation#675.

Remove deprecated APIs (census-instrumentation#667)

Fixes census-instrumentation#494.

Fix gRPC README in examples (census-instrumentation#628)

Don't record measurements with no subscription (census-instrumentatio…

…n#600)

Suggested by Ramon, immediately dropping the measurements from
the measures with no subscription rather than doing it at a later time
significantly improves the performance on the critical path.

Before:
BenchmarkRecord0-8            	500000000	         3.09 ns/op
BenchmarkRecord1-8            	 5000000	       366 ns/op
BenchmarkRecord8-8            	 3000000	       412 ns/op
BenchmarkRecord8_Parallel-8   	 2000000	       804 ns/op
BenchmarkRecord8_8Tags-8      	 3000000	       415 ns/op

After:
BenchmarkRecord0-8            	1000000000	         2.58 ns/op
BenchmarkRecord1-8            	30000000	        36.9 ns/op
BenchmarkRecord8-8            	20000000	        89.4 ns/op
BenchmarkRecord8_Parallel-8   	30000000	        44.8 ns/op
BenchmarkRecord8_8Tags-8      	20000000	        90.1 ns/op

Don't force download all dependencies (census-instrumentation#522)

With census-instrumentation#85,
it became possible to go get the package without ...

Improve godoc for ochttp vars. (census-instrumentation#444)

Also made tag keys agree with trace attributes.

Fixes: census-instrumentation#423

Add Jaeger support to README and fix links (census-instrumentation#383)