Performance Upgrade to _slice

According to http://jsperf.com/arguments-to-array/5, instantiating an
Array with a known lenght is more performant.
Error Correction of _slice
Updated definition of length in _slice
Corrected args to arr in _slice
Corrected length definition in _slice
Finally corrected _slice bug
Added length var
Added semicolon
Merge pull request #70 from cgarciae/master

Speed improvement to _slice