return type of input field (issue #217)

Do not check @type against NON_TEXT_TYPES when returning the attribute
value. Fixes a problem where 'email' was returned as 'text'.
clean up code

- remove unused html
- tidy up comments
- simplify input.type method

Thanks @jarmo for feedback!
remove input.type method

already implemented (thanks @jarib)
fix missing assertion

.should missing (thanks @p0deje)
Merge pull request #218 from bboyle/html5-input-types

return type of input field (issue #217)