Remove GCMNoCollapseKeyException

It turned out that collapse key is not a requirement for messages with
TTL at least since Oct, 2012.

See https://plus.google.com/u/0/104524825852741167674/posts/AEpCusdNZY9
for details.
Coveralls test coverage reporting.
Mock for urllib2.urlopen.
Regression test for UnicodeError in make_request.
Fix for unicode encode bug.
Added generic make_request_plaintext test.

Factored out response value testing from unicode
regression test.
Update gcm.py

Fixed a situation where if the TTL was 0 the if statement would effectively fail.

A TTL of zero means deliver now or never which is necessary for my use case.
Merge pull request #37 from NetworksAreMadeOfString/master

Update gcm.py
Merge pull request #36 from dokterbob/unicode_regression

Unicode regression
Use RST instead of Markdown for README
Merge pull request #40 from spoqa/readme-rst

Use RST instead of Markdown for README
Merge pull request #30 from imankulov/remove_collapse_key_exception

Remove GCMNoCollapseKeyException
Corrects json_request docstring
Added support for dry_run
Merge pull request #43 from vivekfantain/master

Added support for dry_run
Fix "TypeError: unhashable type" when calling json_request

This bug was introduced as part of #43.  `dry_run` was appended to
construct_payload parameters but there was a single instance where
construct_payload was executed and relying on `is_json` parameter's
default value.
Merge pull request #46 from dkong/fix_json_request

Fix "TypeError: unhashable type" when calling json_request
Update README.rst
Merge pull request #42 from orblivion/patch-1

Corrects json_request docstring
Merge pull request #1 from geeknam/master

update