uritemplate: add expression.addToken() to the interface

Signed-off-by: Alejandro Mery <amery@geeks.cl>
uritemplate.Template: rename tmplFoo to exprFoo

Signed-off-by: Alejandro Mery <amery@geeks.cl>
uritemplate.parser: introduce exprStack

Signed-off-by: Alejandro Mery <amery@geeks.cl>
Add .gitignore and .gitattributes

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