Add lodash which can be accessed with _.
Move lodash to arethusa.core

Test files typically don't get the arethusa module, but only the core,
so it's better to have it there.
Start with arethusa.util module
Remove global var arethusaGenerators
Get rid of global var arethusaClasses

Use the `commons` service of `arethusa.util` instead.
Remove global var arethusaLogger

Use `logger` of `arethusa.util` instead.
Start to move arethusaUtil
Merge pull request #456 from latin-language-toolkit/lodash

Use lodash - remove global variables. Fixes #441 & fixes #501