Autogenerating GET url query string using the contents on the parameters dictionary

An easy way to change between GET and POST methods using the same code
structure. Depending the data on parameters and the httpMethod the property urlPath will display the correct url, filling with data the querystring (for GET requests) or the requestBody (for PUT/POST requests). Also added tests.

Starting with the TTURLRequest object:
```
TTURLRequest *request = [[TTURLRequest alloc] initWithURL:@"http://sample.com" delegate:nil];
````

If the parameters dictionary is empty, ```request.urlPath``` will show:
``` 
http://sample.com 
```

If we fill the parameters dictionary
````
NSDictionary* parameters = [NSDictionary dictionaryWithObjectsAndKeys:
                                @"value1", @"parameter1", @"value2", @"parameter2", nil];
[request.parameters addEntriesFromDictionary:parameters];
````
The method ```request.urlPath``` now will show:
````
http://sample.com?parameter2=value2&parameter1=value1
```````

Using the same ```request``` object and changing httpMethod to POST or PUT, ```request.urlPath``` will show:
````````
http://sample.com
````````
And the parameters will be added to de requestBody