Ignoring case when transforming absolute paths to relative ones

When syncing into a case-sensitive fs, paths and filenames using inconsistent case cause a wrong sync.

Dropbox is a case-insensitive fs, so when you have a primary dropbox managed in a case-insensitive fs (hfs), filenames tend to be stored in whatever original case they were, and protracted file renaming can lead to structures made like this:

	/Documenti/
	/Documenti/.localized
	/documenti/Saves/2011-12/2.jpg
	/documenti/Saves/2011-12/3.jpg
	/documenti/Saves/2011-12/2011-12.pdf
	/documenti/Saves/2011-12/darker.jpg

These should be treated the same, regardless of case. Unfortunately, a case-sensitive fs breaks this:

	/store # dropbox-api sync dropbox:/documenti . -n
	!! enable dry run !!
	download /store/../Documenti/.localized
	mkpath /store/../Documenti/Saves
	mkpath /store/Saves/2011-12
	download /store/Saves/2011-12/2.jpg
	download /store/Saves/2011-12/3.jpg

Notice that I synced "documenti", not "Documenti".

Here's it again running debug:

	/store # dropbox-api sync dropbox:/documenti . -Dn
	!! enable dry run !!
	check: ../Documenti/.localized
	remote: 1306241683          0 /Documenti/.localized
	local:           -          - /store/../Documenti/.localized
	download /store/../Documenti/.localized
	check: ../Documenti/Saves
	remote: /Documenti/Saves
	local:  /store/../Documenti/Saves
	mkpath /store/../Documenti/Saves
	check: Saves/2011-12
	remote: /documenti/Saves/2011-12
	local:  /store/Saves/2011-12
	mkpath /store/Saves/2011-12
	check: Saves/2011-12/2.jpg
	remote: 1363357941    1712875 /documenti/Saves/2011-12/2.jpg
	local:           -          - /store/Saves/2011-12/2.jpg
	download /store/Saves/2011-12/2.jpg
	check: Saves/2011-12/3.jpg
	remote: 1363357941    1532957 /documenti/Saves/2011-12/3.jpg
	local:           -          - /store/Saves/2011-12/3.jpg
	download /store/Saves/2011-12/3.jpg

Again, notice the difference in treatment between different case:

	remote: 1306241683          0 /Documenti/.localized # -> /store/../Documenti/.localized

	remote: /documenti/Saves/2011-12 # -> /store/Saves/2011-12

I've replaced the `abs2rel` from `File::Spec` with an implementation using a case-insensitive regex replace, which while much simpler, should cover all reasonable situations.

	/store # dropbox-api sync dropbox:/documenti . -n
	!! enable dry run !!
	download /store/.localized
	mkpath /store/Saves
	mkpath /store/Saves/2011-12
	download /store/Saves/2011-12/2.jpg
	download /store/Saves/2011-12/3.jpg
	download /store/Saves/2011-12/2011-12.pdf
	download /store/Saves/2011-12/darker.jpg