(android) tablets

added detection for (android) tablets

new test: tablet()
works like mobile()

modified mobile(): a android system is not always a mobile device
(read:
http://googlewebmastercentral.blogspot.de/2011/03/mo-better-to-also-dete
ct-mobile-user.html )
test case

test case
test user agent

test user agent
tablet detection

some bugs fixed + test scenario
Fixes one failing tablet test.
Use Test::Most and Test::FailWarnings.
Bumps version to 1.51