Clarify that Tracer.extract can return null

[mabn]

According to #168 in practice the tracer implementations return null when SpanContext is not found in the carrier. This change documents this behaviour.

It also adds a dependency on com.google.code.findbugs:jsr305 but it's optional, it is not listed in the pom and client applications do not need to depend on jsr305.

Compiling the code above with:

javac -Werror -Xlint:all -cp \
/Users/mabn/.m2/repository/io/opentracing/opentracing-api/0.30.1-SNAPSHOT/opentracing-api-0.30.1-SNAPSHOT.jar:\
/Users/mabn/.m2/repository/io/opentracing/opentracing-noop/0.30.1-SNAPSHOT/opentracing-noop-0.30.1-SNAPSHOT.jar:\
/Users/mabn/.m2/repository/io/opentracing/opentracing-util/0.30.1-SNAPSHOT/opentracing-util-0.30.1-SNAPSHOT.jar \
Main.java

procudes no warnings.

[coveralls]

Coverage remained the same at 73.09% when pulling 0ff87b8 on futuresimple:nullable_extract into e000eb0 on opentracing:master.

[pavolloffay]

Can we do this in a separate PR? It might apply also at different places

[mabn]

Do you mean to split adding jsr305 and marking Tracer.extract into two separate PRs?

[mabn]

I'll close this PR as I haven't looked into this for a while.

[whiskeysierra]

I'd still like to see this happening, tbh.