Use value receiver for config.NewTracer()

[yurishkuro]

The new function is defined on a pointer received, making it harder to use, e.g. below doesn't work even when using the &:

	tracer, closer, err := &jaeger.Configuration{
		ServiceName: *name,
		Sampler:     &jaeger.SamplerConfig{Type: "const", Param: 1},
	}.NewTracer()

Also correct the naming of examples functions to match godoc guidelines, and fix README examples.

[codecov]

Codecov Report

Merging #283 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #283   +/-   ##
=======================================
  Coverage   86.27%   86.27%           
=======================================
  Files          55       55           
  Lines        2972     2972           
=======================================
  Hits         2564     2564           
  Misses        287      287           
  Partials      121      121

Impacted Files	Coverage Δ
config/config.go	92.91% <100%> (ø)	⬆️

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