Fix ambiguous declaration.

Cocos2d-x also have HttpClient class. When integrating with cocos2d-x, compiler can't determine exact class type.
* Async callback (errors are not yet managed properly)
Add const keyword, use references, coding style

* Methods which doesn't change an attribute should be set as const.
* Pass arguments as reference to avoid copy.
* Line length: 80
Constructor style
* Error handling for callbacks
Public github repo url in the README
Merge pull request #23 from MiLk/fixes/repo-url

Public github repo url in the README
Merge pull request #20 from stelcheck/features/async-callback

(WIP) Have the option to do async through callbacks instead of futures
Merge branch 'develop' into fixes/ref-const-style
Merge pull request #18 from tokutake/fixAmbiguousDeclaration

Fix ambiguous declaration.
Exceptions improvements

* use const keyword
* use an enum instead of a string to know the type of the exception
* line length
* remove throw in destructor (was removed with C++11)
* type should not be changed after initialization
* use virtual keyword
Remove unnecessary output
Pass string by reference
Merge branch 'develop' into fixes/ref-const-style
Code style
* Lint now lints examples
* Max line lenght is 120
* Everything lints
* Fix: Call(callback) was not running in async
* Fix: Example for callback now demonstrated an async behaviour
Merge branch 'develop' into unity
Use a UNITY preprocessor define to build for Unity