Add tests for session storage constructors
Merge pull request #81 from popthestack/master

Do not overwrite OAuth session if already present.
Merge pull request #82 from gigablah/session-test

Add tests for session storage constructors
improve readme with packagist badges and composer info
Merge pull request #84 from toin0u/improve-readme

improve readme with packagist badges and composer info
Add `Common\Http\Client\AbstractClient`.

`Common\Http\Client\StreamClient` should extend the new `AbstractClient`.
Remove `maxRedirects` and `timeout` from constructor and set default directly on class.
Add setters for `maxRedirects` and `timeout`.
Move `normalizeHeaders` to `AbstractClient`.
Add `Common\Http\Client\CurlClient`.
Add `ServiceFactory::setHttpClient()` class method.

 * Remove `ServiceFactory::__construct`. HTTP client should be set via
`setHttpClient`.
 * Remove `$httpClientMap` as it's not longer necessary. `setHttpClient` enforces
that the client implements ClientInterface via type hinting.
 * To maintain backwards compatibility, `ServiceFactory::createService` creates a
`Common\Http\Client\StreamClient` instance if no HTTP client has been set.

I'm not crazy about the backwards compatibility hack, but it keeps the library
functioning the way it did before while at the same time adding flexibility.