Implement indentation for SPARQL expressions

This is a very simple indentation function for the sparql-mode. 

It works for the things I want and the indentation level is customisable. I set it to 2 as a standard, but I can change that if wanted.

It also indents inside `()`, but instead of following the indentation level it forces the indentation to the column after the receiving `(`. If you don't like that I can change it to be configurable as well.