preserve quoting in attribute values

[maditya]

@adon-at-work @yukinying @neraliu please review!
This is to address #17

[adon-at-work]

my two cents are:

• type of attrVals is converted from object to array. therefore, <a href="123" sth=1 href="abc">hello</a> will be converted as <a href="123" href="abc">hello</a> as output, whereas the old implementation will generate <a href="abc">hello</a>. i prefer the latter output.
• when key === "style", double quote is still used.
• given the original implementation this.attrVals[parser.getAttributeName()] = parser.getAttributeValue();, i thought we could store the value with the appropriate quote (instead of the value), i anticipate the impl can be much cleaner and faster.

not sure whether i explain it clear enough here. let me make a commit to address what i proposed here.

[yahoocla]

CLA is valid!