Handle case-sensitivity, fixes #240

The result of fs/absolute-path might differ in case. Result of
getCanonicalPath is defined to "be both absolute and unique" [1]. As
noted by Chas, in #240, this probably is an incremental solution only.

[1] http://docs.oracle.com/javase/7/docs/api/java/io/File.html#getCanonicalPath()
Merge pull request #241 from Ekahau/fix/case_sensitivity_in_relativize

Handle case-sensitivity, fixes #240