Support excel files in tabular module

[blcham]

Notes:

• each sheet will be represented as a separate "table entity", see Initial simple implementation of the xls files support in TabularModule #203 (comment)

A/C:

• within output RDF it is possible to query which columns or rows where merged
• there is optional parameter to process only one sheet
• functionality is tested with JUNIT tests including test with multiple sheets
• module should support parsing multiple CSV files as well
• if implemented using property holding mime-type value, it should be checked that combination of delimiter value and mime-type values are together consistent
• TabularModule main documentation is revised by @blcham (especially default behaviour, process-specific-sheet-in-xls-file should be refactored to address multiple tables, is it ok to replace cell value '\t' to ' ' in HTML and excel? )

Should be implemented after #204

[blcham]

FYI, regarding merged columns and rows -- this is how export to CSV works in google drive:

[blcham]

Regarding parsing Excel files, consider (don't forget about merged cells/columns):
https://sourceforge.net/p/supercsv/discussion/718794/thread/b85ce454/
http://techs.studyhorror.com/d/apache-poi-how-to-convert-csv-to-excel-using-superc

[blcham]

@rodionnv I have added requirment to "module should support parsing multiple CSV files as well" (it is basically same as support for multiple excel sheets)