Adding initial version of parser trait

Added parse_chunk method declaration to parser

Removed unnecessary visibilty for parse_chunk function

Implemented parse_chunk function

Implemented parse_chunk fn for ServoHTMLParser

Moved parser trait to mod.rs and added finish fn

added licence header to mod.rs and other review comments

Fixed trailing space issue

Fixed failed tabular space test
auto merge of #3718 : juzer10/servo/master, r=jdm

We have created parser trait and declared parse_chunk function in this trait. We are yet to implement this parse_chunk for ServoHTMLParser struct.