Create an HDT in one pass.

[mielvds]

This PR is preliminary work to fix #47 and is not at all finished. However, I hope this starts a discussion that will lead to a possible solution. This is loosely based on what was implemented for Java.

General approach:

• fix PlainDictionary.cpp to return inserted IDs.
• add a loadOnePassmethod to BasicHDT.cpp to combine/replace loadDictionary()and loadTriples() which
  • builds a temporary dictionary
  • encodes all triples with IDs given by that temp. dictionary
  • create the final dictionary
  • update all triple IDs according to the new dictionary
• update rdf2hdt.cpp to supply a 'loader type'

This is not at all performant yet, as my C++ knowledge is also so so. Therefore, I hope to receive enough help and feedback through this PR :)