Semantic citations enabled; plus minor improvements. v0.3-2

* Semantic citations now enabled.  `bibliography` can print with method 'rdfa'
  to provide RDFa enhanced HTML bibliography data, and citations can take cito
  arguments as an option. (Closes issues #16 and #17)
* `bibliography` now takes more options to match the `bibstyle` and `print.bibentry`
  functions.
* `citet` and `citep` take an optional argument for formating the citation, defaulting
  to `authoryear` format.  Future versions may move this into an option.
* `newbib()` function initializes and clears existing cache files.
* README.md updated with semantic examples and more background text (generated by
  inst/examples/README.Rmd).
* Some basic unit tests built in.  Could use more, but meanwhile closes #8.
* Test cases work with knit2html button in RStudio, closes #14