Add implementation for string callback
Add implementation for start_map callback
Fix: Name of a command must be assignable.
Add implementation of map_key callback
Add implementation of end_map callback
Add implementation of start_array callback
Add implementation of end_array callback
Slightly adapt documentation

The current implementation does not allow

    { "bool": true }

objects as arguments, but gives the posibility to pass "true"/"false"
and "null" (which translates to NIL internaly) directly. This patch
adapts the documentation on this behaviour.
We do not set the number callback

We don't set it because yajl uses the integer/double callbacks if we do
not set it, and these are preferable.