Verify that pmap holds head of results

This test verifies that pmap is holding the head of the results
sequence, potentially leaking memory.

see also http://clojure.org/lazy
Add tests to all lazy functions
Fix potential memory leak head holding
Remove unnecessary Java class
Add detailed note about GC assumptions in test
Less sleeping in GC test
Merge pull request #15 from TheClimateCorporation/head-holding-test

Fix lazy-seq-head-holding memory leak