uritemplate: reorganize code

Signed-off-by: Alejandro Mery <amery@geeks.cl>
uritemplate: refactor Template.addToken() expending exprSequence.addToken()

Signed-off-by: Alejandro Mery <amery@geeks.cl>
uritemplate.Template: add exprOptional "[" ... "]" support

Signed-off-by: Alejandro Mery <amery@geeks.cl>
uritemplate: remove unneeded exprSEQUENCE type

Signed-off-by: Alejandro Mery <amery@geeks.cl>
uritemaplte: print exprEOL as $

Signed-off-by: Alejandro Mery <amery@geeks.cl>