explicitly some instance variables via PsychHandler#initialize

It removes below ruby's warning.

* instance variable @{ivar} not initialized
Merge pull request #2 from kachick/improve-remove_warnings

Explicitly some instance variables via PsychHandler#initialize
renamed #orig_load to #unsafe_load, updated #load signature to accept a :safe flag
fixed specs for Ruby 1.9.2
fixed specs for Ruby 1.8.7
refactored default options into SafeYAML module, added methods to toggle default behavior (safe more or not)
silenced warnings for cleaner spec output
updated #load_file signature to accept :safe flag
fixed up/fleshed out some specs
updated version and README file