Remove notice when get returns null

get member function returns now a reference to an empty array variable
in order to avoid the notice "Only variable references should be
returned by reference" when returns null if there is no results.
add and set functions return a boolean

add and set member functions return now a boolean to indicate
whether or not the operation succeeded.
Merge pull request #3 from martinec/master

Update JsonStore
- Return empty array instead of null on get()
- set() & add() return a boolean