Finish with the warning on launch

Hello,

I hava a small problem. When i use your gem (thanks for it), i get theses warnings.

/home/sebastien/.rvm/gems/ruby-1.9.3-p392/gems/i2c-0.2.0/lib/i2c/drivers/mcp23017.rb:16: warning: already initialized constant INPUT
/home/sebastien/.rvm/gems/ruby-1.9.3-p392/gems/i2c-0.2.0/lib/i2c/drivers/mcp23017.rb:17: warning: already initialized constant OUTPUT
/home/sebastien/.rvm/gems/ruby-1.9.3-p392/gems/i2c-0.2.0/lib/i2c/drivers/mcp23017.rb:20: warning: already initialized constant HIGH
/home/sebastien/.rvm/gems/ruby-1.9.3-p392/gems/i2c-0.2.0/lib/i2c/drivers/mcp23017.rb:21: warning: already initialized constant LOW

This small commit disable these messages.

Good afternoon