[BUGFIX] Building and showing mapping information breaks

If no Indexable annotations are used, the use of a NULL value in a
foreach loop would break the code.

Also the mapping:showstatus would break on "complex" mapping settings
that are not simple strings.
[TASK] Minors fixes in documentation
[!!!][TASK] Change transformer interface to use non static methods

Currently the getTargetMappingType and transformByAnnotation are static
method. This patch change this to allow more complex transformer that need
DI.

This change is breaking if you use custom transform class. In this case
you just need to adapt the previously static methods.
Merge pull request #4 from Flowpack/bugfix

[BUGFIX] Building and showing mapping information breaks
Merge pull request #5 from ttreeagency/feature/DocumentationFixes

[TASK] Minors fixes in documentation
Merge pull request #6 from ttreeagency/feature/NonStaticTransformer

[!!!][TASK] Change transformer interface to use non static methods