Various improvements.

* Expand README.md, provide benchmark results.
* Add docs, benchmarks for binheap and circ packages.
* Add methods Len() and Capacity().
* Change *sync.Cond to sync.Cond.
* TryRecv() and TrySend() distinguish empty and closed errors.
* Improve test coverage.
* Add basic Makefile.
* Fix documentation mistakes.

Makefile

@@ -0,0 +1,11 @@
+.PHONY: test
+test:
+	go test -count=1 -coverprofile=coverage ./...
+
+.PHONY: bench
+bench:
+	go test -bench=. ./...
+
+.PHONY: longbench
+longbench:
+	go test -count=30 -bench=. ./... | tee benchresults.txt