bugfix: 'kid' not in given key list

The README says this package is conform rfc7519 (May 2015) rather than just the 6th draft (valid until June 2013). The implementation seems indeed to be compatible with the rfc.

if 'kid' value is not found in the given key map, should throw an exception.
Instead, it was outputting a php warning for using an undefined index, resulting in a null key.

Add in support for late static binding, so that developers can extend and override variables / methods if they so require (#88)

* Use InvalidArgumentException when $allowed_algs is not array

> Exception thrown if an argument is not of the expected type.
http://php.net/manual/en/class.invalidargumentexception.php

* Use RuntimeExceptions for exceptions related with unencoded data.

RuntimeExceptions is the correct exception error source is the decoded data.

Note LogicExceptions as defined in PHP documentation implies a modification in the code by the developer.

> Exception that represents error in the program logic. This kind of exception should lead directly to a fix in your code.
http://php.net/manual/en/class.logicexception.php

But the token is a data provided by an external source which is out side of the control of the developer so there is no way of prevent malformed tokens.

".gitattributes" <- prevent installing special files via composer

Use static $timestamp instead of time()

Remove newline at beginning of run-tests.sh

Remove null as possible key value in docblock

The extra newline trips a warning in lintian when packaging for Debian,
so just remove it.

[*]: add missing phpdoc

The docblock states that null is a possible value for $key, however, it clearly results in an exception.

Update Travis to use their new container infrastructure

See [here](http://docs.travis-ci.com/user/migrating-from-legacy/) for details.

require a non-empty key to decode a JWT

Aligned the DocBlocks

Clarified and corrected the documentation where needed.

Add firebase\jwt namespace