Object representation improvements

[marinagrprat]

Problem

When we call the object pyxnat it returns just a collection object and an ID number.

Feature

We want to return more information (metadata information) from the object. Some metadata information examples are the age, gender, handedness, insert data, insert user, number of subjects, experiments, scans...etc.

Solution

The changes has been done in resources.py file.
A repr method has been added to each class (Project, Subject, Experiment, Scan and Resource)
Each method is capable to return the main information for each object dynamically (will just appear the information that the obejct contains, the empty fields will not appear) and a link that redirects you to the XNAT page where the object is.
This method allows us to know more information about the object immediately and find out more just clicking the link.

Unit Tests

For each object, 3 tests cases has been added. So in total we have 15 unit tests in a new file tests/repr_test.py.
The tests cover if the object exist, if the object does not exist and if it exists that the information that returns is correct.

[marinagrprat]

This PR has been replaced for: #141