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Added method getTtl to get the time when a key expires. See #49

• Added option errorOnMissing to throw/callback an error o a miss during a .get( "key" )
  Thanks to David Godfrey for the pull #45
• Added docker files and a script to run test on different node versions locally

• unref() the checkperiod timer
• optimized travis.yml

changed return format of .get() with a error return on a miss and added the .mget() method. Side effect: Performance of .get() up to 330 times faster!

This Release is planed to be published to NPM until Feb. 2015.

added .keys() method to list all existing keys.

fix for setting numeric values. Thanks to kaspars + optimized key ckeck.

Small change for better ttl handling

Made callbacks optional. So it's now possible to use a syncron syntax. The old syntax should also work well.

Added events set, del, expired and ´flush´ to nodecache to react on chaning data.