Refactoring

This refactoring consists of these changes:
* functions download, downloadFile and class ChecksumMode moved to maven_repo_util and removed function fetchFile (no need to have two functions that do the same thing), also the signatures of those functions were changed a bit
* functions setLogLevel, str2bool, slashAtTheEnd, getRegExpsFromStrings and somethingMatch simplified
* Imports refactored to follow this format:
```
    import external_lib_1  # e.g. sys, os, urllib2, re...
    import external_lib_2
    ...
    from external_lib_1 import external_member_1  # class, function, constant ...
    ...
    
    import local_package_1  # e.g. maven_artifact, maven_repo_util ...
    import local_package_2
    ...
    from local_package_1 import local_member_1
    ...

    # Code
    def ...
```

Please review :)