Why is the title To Kill a Mockingbird full of significance and who is the mocking bird?

Many characters within the book are 'Mockingbirds' (Although the main one is Tom Robinson). The mockingbird was supposed to represent inosence and the title also refers to Atticus' quote ".....remember it's a sin to kill a mockingbird."

If you reword the quote

".....remember it's a sin to kill an inocent man"

Mockingbird = Tom Robinson = Inocent man

The entire quote gives away the story plot if looked at carefully. 

"Shoot all the blue jays you want, if you can hit 'em, but remember, it's a sin to kill a mockingbird."

The bluejay represents the bad people (eg. Bob Ewell) and how he didn't get caught for the crime he obviously commited.

The entire quote reworded:

"Arrest all the bad people you want, if you can CATCH them, but remember, it's a sin to kill and inocent man."