Changed the syntax from 5.2 to 5.3 using namespace

Closes #223
Changed all syntax using namespaces, in compliance with PSR-2.
Ex: `Elastica_Type` becomes `Elastica\Type`
**WARNING**: Very heavy BC break, I strongly suggest to create a branch for the old syntax

### Composer
* Usage of composer for `lib` and `test` autoloading
* Added `PHPUnit` as a dev dependency in `composer.json`

### Tests
* All tests were rewritten for new syntax.
* All tests where moved in `Elastica\Test` namespace
* All tests now inherit from `Elastica\Test\Base`
* There is currently an issue regarding the code coverage, see sebastianbergmann/php-code-coverage#145

### Misc
* Removed all executable flags on files where not needed.