Use '.' instead of '=' in URLSafeBase64Encoder.

According to [RFC 3986](http://www.ietf.org/rfc/rfc3986.txt) section 2.3, the equal sign (=) is not an
unreserved character in URLs:

> Characters that are allowed in a URI but do not have a reserved purpose are called unreserved.  These include uppercase and lowercase letters, decimal digits, hyphen, period, underscore, and tilde.

Since the tilde is common used to represent home directories, I chose to use the period (.) for padding instead.