Update README.md

Tried to improve "Standalone with ActiveRecord" section.
more README tunning
[DB2] Fix error with timezone
[DB2] Use default active record date and time parsing
[DB2] enable tests for AS400
[DB2] fix mocha version
[DB2] remove mistake with 2.3 compatibility
Merge pull request #429 from pierrickrouxel/master

  [DB2] remove mistake with 2.3 compatibility
  [DB2] fix mocha version
  [DB2] enable tests for AS400
  [DB2] Use default active record date and time parsing
  [DB2] Fix error with timezone