Clarified i vs l in serial input

The spark docs font does a bad job at distinguishing between "i" and "l".
 I spent a bit of time myself not realizing this was an "i", so I hope
 this change will stop this from happening to people in the future.
Added instructions for serial connection w/ screen
added note about max len of Spark.variable names
tweaks + minor rewording for pull request #99
Merge branch 'master' of https://github.com/blakefrantz/docs into blakefrantz-master