Modified env resource to break values up by delimiter before comparing.

For example, if someone were to do before this patch,
```ruby
windows_path 'C:\foo;C:\bar' do
  action :create
  ...
end
```
the path would grow each time this recipe was executed. This is because
compare_path would break up the current path with ;, but would not do
the same for the value being added.
Added unit tests to ensure that value for env correctly deals with having the delimiter in it
Modified spec to check subpath case
Got rid of repeated code
delete_element should return false if the key needs to be deleted
Renamed compare_value in env provider
Aliased compare_value to requires_modify_or_create?
Sytle cleanup
Merge pull request #2252 from opscode/jdmundrawala/env-split

Modified env resource to break values up by delimiter before comparing