Use appropriate command invocations for rails tasks.

Standardize calls to generate, console, destroy etc to use the first
version found of `script/$command`, `script/rails $command` or `rails
$command`.

This means those commands now work again in Rails 3+ and with RVM if
you use `rvm.el` with `rvm-activate-corresponding-ruby`.
Run default webserver with correct invocation.
Allow script to be ran when visiting a file in the project; current working directory does not always reflect project root.
Merge pull request #9 from remvee/rails-scripts

Use appropriate command invocations for rails tasks.