Create an input which generates a datalist tag

Create an input which supports http://www.w3schools.com/tags/tag_datalist.asp
Used as follows
f.input :fieldname, as: datalist, collection: ["list", "of", "items"]
collection option accepts all formats supported by rails' options_for_select
Add documentation for datalist input

Add an example in README
Fix scope of DatalistInput class
Add documentation for DatalistInput class
Minor fix for file_input documentation
Add tests for datalist input

Add tests for both kinds of collections that can be provided
Add more generic helpers in custom_macros, which can be better reused
Add datalist input to autoload list
Remove invalid TODO
Improve intent of the regex used to strip out _id, and other PR fixes
Merge pull request #1051 from ktaragorn/datalist_input

Add a new DatalistInput (:as => :datalist)