Fix for #204 - line_comment_prefix

Fix for this issue: https://github.com/mitsuhiko/jinja2/issues/204

This fix cascaded into a few other tests when it comes to whitespace. 

For `test_line_syntax_priority`, since this line was in the code:
```
# XXX: why is the whitespace there in front of the newline?
```
I feel that it is alright for me to change that test.

I'm not sure about `test_lstrip_angle_bracket` and `test_lstrip_angle_bracket_compact`. On line 505:
``` 
${item} ## the rest of the stuff
```
It seems like there could be a whitespace before the comment, but I'm not sure.

Let me know if I should change anything!