Implementing "-I" command-line flag to add to $LOAD_PATH
Handling path separators in -I arguments

plus adding the missing `File::PATH_SEPARATOR` const even though I
decided to just reference the same python var rather than pulling the
const definition back out of the object space.
Breaking apart tests per feedback

and consolidating load path appends into a single for loop.
Merge pull request #349 from modcloth/load-path-flag

Adding support for "-I" `$LOAD_PATH` flag