Enable usage of a 'per execution' lexicon file.

This pull request replaces #101 and #103.

An example request is as follow:
```xml
<?xml version="1.0" encoding="UTF-8"?>
<maryxml xmlns="http://mary.dfki.de/2002/MaryXML" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="0.5" xml:lang="it" lexicon="/custom_dict_path/abituarci.lexicon">
<p>
abituarci
</p>
</maryxml>
```
The content of the lexicon file (/custom_dict_path/abituarci.lexicon) should be in the format "form | phonetic transcript | pos":
abituarci | a - b i - t u - ' a1 r - tS i | V