Map out some new API stuff in the README
First crack at extending the API
  * add the Converter type
  * add a WriteString() function for easier testing
  * try (but fail) to add the ability to set custom headers
Fix little booboo in README
Remove dead printf
These function definitions don't need to be against pointers.
Get String()/WriteString()/Headers all working properly.

Thanks @Mtibben for pointing out where I confused myself.
Push WriteFile() onto the Converter too.
Add ability to not write a header row with WriteHeaders flag.
Remove temporary and unhelpful test.
Add the ability to preprocess your rows.
Clean up tests a little bit.
Fix some style and error handling issues.

Thanks @davecheney for the review!
Allow TimeFormat to be set for conversions.
Make the Converter struct more godocable.
Link off to godocs for more of the advanced settings.
Merge pull request #1 from joho/add-converter-type

Add Converter for doing Advanced Shit™ in your CSV conversion