Add optional directory parameter

The optional <directory> configuration parameter (#6) makes it possible to
restrict where docbook-linktester looks for files.

This is an alternative to using <excludes> when source files have raw Maven
properties to filter or share names with files to exclude, such as
src/site/xdoc/index.xml.

Tested with forgerock-doc-maven-plugin-test
Simplify logic for directory handling

Ongoing work towards #6
Conform with coding style guidelines
Improve naming of utility classes
Add multivalue support for documentation file selection

This is to fully resolve #6, the following changes were introduced:
* Moved to maven-plugin-plugin and leveraging now the Maven annotations
across the Mojo. This also gave a good opportunity to enable generation of
help Mojo as well.
* Added a few missing <scope>provided</scope> to dependencies
* Improved initialization and error handling in general
Log a warning for >=400 response codes
Don't set TrustAll SSLSocketFactory globally

Resolves #9
Use final keyword wherever possible
OutputFile should be saved per module

Resolves final drop out from #8
Update Maven dependencies
Update developer info :)
Update the README based on the recent changes