update maven-assembly-plugin

- use version to 2.4
- build assembly now with command
  mvn package  (assembly:assembly is deprecated)
- adjusted README.md
added UDF to normalize link targets to DBpedia URIs
added macro file that contains functionality of main scripts

will be used by (at least)
- examples/indexing/names_and_entities.pig
- examples/nerd-stats/nerd-stats.pig
scripts import macros

- nerd-stats.pig and names_and_entities.pig both use macros
- deleted examples/indexing/names_and_entities_low_memory.pig
deleted NGramGenerator in favor of RestrictedNGramGenerator

RestrictedNGramGenerator can be initiated with '' for the surface forms file,
which results in similar behavior as NGramGenerator, but with Spotlight
tokenizers.

*BUT* the behavior is different: see failing test!
re-factored RestrictedNGramGenerator

- variables to refer to distributed cache files inside and outside of getCacheFiles()
- Bugfix: loading of tokenizer model with correct variable and in exec()
- some formatting
added Travis-CI config

Fixes https://github.com/dbpedia-spotlight/dbpedia-spotlight/issues/172
use nerd_commons macros in token_counts.pig
formatting in token_counts.pig
Fix TestNGramGenerator.

Use English OpenNLP that is already in resources/.
Had to add protected constructor to RestrictedNGramGenerator.
Merge pull request #8 from maxjakob/macros

De-duplication using macros

Fixes #5.
fix oversight in RestrictedNGramGenerator
bugfix

forgot to define UDF
add version of maven-compiler-plugin

3.1
aliases may not start with an underscore

Thanks to @areggiori for reporting.
https://github.com/dbpedia-spotlight/pignlproc/pull/10#issuecomment-19822399