LinkDB: move to a proper file, add test coverage

LinkDB: move to a proper file, add test coverage

Relates to #71

LinkDB
 - move to application/LinkDB.php
 - code cleanup
   - indentation
   - whitespaces
   - formatting
 - comment cleanup
   - add missing documentation
   - unify formatting

Test coverage for LinkDB
 - constructor
 - public / private access
 - link-related methods

Shaarli utilities (LinkDB dependencies)
 - move startsWith() and endsWith() functions to application/Utils.php
 - add test coverage

Dev utilities
 - Composer: add PHPUnit to dev dependencies
 - Makefile:
    - update lint targets
    - add test targets
    - generate coverage reports

Signed-off-by: VirtualTam <virtualtam@flibidi.net>