replace List with ArrayBuffer for MoeArrayObject

Scala Lists are immutable.  Change to use ArrayBuffer as the underlying
implementation to support more perlish array operations
array length method + tests
add "clear" method for arrays
array.shift + tests
array.pop + tests
unshift for arrays + tests
array.push + tests
return undef for non-existent Array index lookups
array slice support + tests
array reverse + tests
array join + tests
Merge pull request #66 from brianphillips/arraybuffer-for-arrays

native MoeArrayObject methods