Truncated page var; .Site.Params; First function in templates

Three changes I've made to hugo while exploring migrating my site to it:

 1. Add a `.Truncated` var to each page, indicating that `.Summary` is truncated from the full text, so that a "more" link can be conditional instead of having "click here for more ... oh, that was it"
 2. Provide for a `.Site.Params` equivalent to page `.Params`; I'm not fixed on the name in the config file (`siteparamsbag`) but I definitely want this functionality, so that I can pull various mutating bits out to the top-level and write my layouts to reference these site variables
 3. Rather than create custom slice copies of lists for new constrained lists, add a `First` function to the template parser, which can return the first "up to N" items from a list, letting me use `{{range First 5 .Site.Recent}}` in a sidebar.