Standardise use of CanonicalPath

Location.scala uses AbstractFile.canonicalPath while
ScoverageHtmlWriter.scala uses java.io.File.getAbsolutePath for file
paths. This mean that there is a possibility (esp. Windows environment)
that creation of reports will fail because the scoverage.coverage.xml
created will contain paths with an upper case drive letter while
ScoverageHtmlWriter will be using paths with a lower case drive letter.

Addresses issue #74