Revert "Removed apostrophes"

This reverts commit 39e5504d9b470c386fe412108b67a85ed6c0dba3.
Add likely.h

This adds the likely() and unlikely() macros for branch condition
optimization.
Merge pull request #23 from matthiasbeyer/add-likely_unlikely_macros

Add likely.h
Merge pull request #22 from matthiasbeyer/revert-wrong_typo_fix

Revert "Removed apostrophes"
Add structure to section about operations
Explain insertion of items
Explain serching/finding operation
Explain determination of union of sets
Explain intersection
Explain exclusion