Make conversion to CSV files unnecessary

Addresses my comments from issue #81.

This pull request causes swirl to read content directly from Rmd files, bypassing (the unnecessary and innefficient) conversion to CSV files. CSV files are also capable of being read, but we no longer need two different copies of the same lesson (Rmd + CSV).

`parse_content()` dispatches to the appropriate method based on the file extension of the file called `lesson` in the chosen lesson directory. Methods are then responsible for parsing the content in it's native format and returning an object of class `lesson`.

One tremendous advantage of doing things this way is that YAML from Rmd files is converted directly to `lesson` attributes at runtime. CSV attributes are limited to lesson and course names at this point.

Comments welcome.