JENA-551 Inconsistency between assembler usage for tdb:location and text:directory

This pull request changes the logic in the TextIndexLuceneAssembler#open method, allowing literals for text:directory, mimicking the behaviour of tdb:location. 

The issue hasn't been discussed in the mailing list or in JIRA, so it is all right if this PR is not merged. 

While the fix allows directories to be created using literals, it also adds the "special case" when a literal is equals to "mem" (so it uses a RAMDirectory). What is not very elegant.

If this pull request looks good and gets merged, I can update the site to reflect the changes as well.