Split find/find_all apis

[josh]

Trail#find was amended in 94d6727 to take a block and return all matching results. Its okay, but it overloaded find. Really this should have a been a separate method.

This splits that behavior into Trail#find_all. And now that we are at least on 1.9.3, we can take more advantages of the enumerator api additions.

# always returns immediately with enumerator
trail.find_all("application.js")

# same as find(), just grab the first match
# enumerating is lazy, so its just as efficient 
trail.find_all("application.js").first

# in Ruby 2.0 we can use lazy
trail.find_all("application.js").lazy.select { |path| path =~ /custom constraint/ }.first