Make code generation more yard friendly (for #215)
improvements to YARD macro and attribute handler

1. replace YARD handler with YARD macro
2. explicitly define IDL attribute on class attribute
3. prefer IDL attribute over content attribute
4. removed ‘#html_for’ method
5. deprecate ‘#colspan’ and ‘#rowspan’ methods
attr is now string, so IGNORED_ATTRIBUTES should be string too
method to inherit attributes from class
generated IDL attribute should be symbol, not string
deprecate #readonly? in favour of #read_only?
revert deprecation of #readonly?, #colspan and #rowspan
use constants in generated attributes when possible
Update from HTML5 spec
Generated code should be indented by 2 spaces
Do not generate duplicating classes
Ignore some attributes from html spec
Use constants in manually defined attributes
No need to special case :for attribute
Do not generate methods for ignored attributes
Merge pull request #261 from watir/yard-friendly-code-generation

YARD friendly code generation