Update: Add getSourceLines() to core and rule context (fixed #1005)

Added getSourceLines() to eslint.js; didn't include option to pass node and return only source lines of given node. Replaced getSource() with getSourceLines() where applicable.

Changed error message from no-mixed-spaces-and-tabs rule: removed "Line n has ..." because it's redundant.
I think we probably need to freeze this array so it isn't accidentally changed by a rule.