Memoize objects

[michaelbaudino]

We should memoize already downloaded objects.
For example, the following snippet should execute only one request to the API:

TimestampAPI::Project.find(123)
TimestampAPI::Project.find(123)

Also, the following snippet should require project 123 only once:

task = TimestampAPI::Project.find(123).tasks.first
task.project

We already do it on an object (calling task.project twice on the same object only trigger one API call), we should do it globally.

Objects could thus be shared, this would be great in the end:

project1 = TimestampAPI::Project.find(1)
project2 = TimestampAPI::Project.find(2)
project1.client === project2.client # => true