Enable using dots and underscores in usernames

Let's enable using dots and underscores in the user names. Those 'special'
characters should only be present in the middle of usernames, so
'john.doe' is valid, '.johndoe' is invalid.
Keep original file name when importing files
Add Localization::activeLocale()

We should determine the active language dynamically.
So, let's introduce Localization::activeLocale() that should be used
instead of LANGUAGE
Add file/directory existance checks to grunt

Add functions to simply check if a file and/or a directory exists.

Example:
var c5fs = require('libraries/fs');
var isDirectory = c5fs.isDirectory(path);
var isFile = c5fs.isFile(path);
var isFileOrDirectory = c5fs.fileExists(path);
Reverting always showing installation languages in English
Merge branch 'multibyte-file-importer' of https://github.com/hissy/concrete5 into hissy-multibyte-file-importer
Merge pull request #1541 from mlocati/username-with-dots-and-underscores

Enable using dots and underscores in usernames
updating user error message
Merge pull request #1618 from mlocati/add-localization-activelanguage

Add Localization::activeLanguage()
Merge pull request #1620 from mlocati/add-useful-grunt-functions

Add file/directory existance checks to grunt