- shorten sleep time for speedup test

so anyone can hack ttl without rebuilding ttl from source

Merge pull request #68 from wuwen5/fix-java8-lambda

Utility classes should not have public constructors

- update fuction description of README
- add toc of README
- link to javadoc 7 instead of javadoc 6

- update code and doc: concept multi-thread context(MTC) => TTL
- package mtc => ttl
- update word 'context' to 'TTL value'

- add throws javadoc
- more description about idempotency
- explicit describe that return null when input is null