Add RuleContext.filename property (for eslint/eslint#468).
Document optional filename and default it to '<input>'.

Adds test for new default filename value.
Update API to use context.getFilename() instead of .filename.

Reverts method signature change to RuleContext since the filename is
now available in the eslint object.

Also reverts addition of filename property on RuleContext and replaces it
with passthrough getFilename() call.
Merge pull request #686 from lsegal/add-filename-to-rules

Add RuleContext.filename property (fixes #468)