Clean up documentation, error message

1) Gadfly.jl had an error message that you can’t use global elements
inside a layer, but the error message said “can” instead of “can’t”

2) Fixed all the RDatasets example changes where dots and lower case
letters were switched

3) Made minor grammar and spelling changes to index.md
Merge pull request #169 from randyzwitch/master

Clean up documentation, error message