Bug fixes: changed items over-broadcasting

• [Bug] Changes are no longer broadcast repeatedly with the same items
• [Test] Increased code coverage in YMMemoryCache.m to 100%
• Travis passes more often (Github API)

Tightening up

• Tightened up notification/eviction timing and timer setup.
• Improved the README with examples.
• Increased code coverage in YMMemoryCache.m to 99%.

Initial Release

Merge pull request #3 from yahoo/fix/readme2

Polished README, Add Mantle example note, proper license