[Backend] Add the trees controller

[zergov]

Closes: https://github.com/visevol/GihubVisualisation/issues/42

This controller can be used to find all files on a repository, with their line numbers.

Usage:

GET /repositories/3/tree/head

returns:

[{"filepath":".formatter.exs","line_count":4},{"filepath":".gitignore","line_count":24},{"filepath":"LICENSE.txt","line_count":21},{"filepath":"README.md","line_count":32},{"filepath":"amqp/amqp-xml-doc0-9-1.pdf","line_count":0},{"filepath":"amqp/amqp0-9-1.pdf","line_count":0},{"filepath":"amqp/amqp0-9-1.stripped.xml","line_count":459},{"filepath":"amqp/amqp0-9-1.xml","line_count":2843},{"filepath":"config/config.exs","line_count":30},{"filepath":"examples/.gitignore","line_count":1},{"filepath":"examples/README.md","line_count":12},{"filepath":"examples/requirements.txt","line_count":1},{"filepath":"examples/tutorial_1_hello_world/receive.py","line_count":18},{"filepath":"examples/tutorial_1_hello_world/send.py","line_count":12},{"filepath":"lib/venomq.ex","line_count":19},{"filepath":"lib/venomq/channel.ex","line_count":152},{"filepath":"lib/venomq/channel_supervisor.ex","line_count":16},{"filepath":"lib/venomq/connection.ex","line_count":137},{"filepath":"lib/venomq/connection_acceptor.ex","line_count":27},{"filepath":"lib/venomq/exchange_direct.ex","line_count":49},{"filepath":"lib/venomq/exchange_supervisor.ex","line_count":27},{"filepath":"lib/venomq/queue.ex","line_count":94},{"filepath":"lib/venomq/queue_supervisor.ex","line_count":51},{"filepath":"lib/venomq/transport/data.ex","line_count":136},{"filepath":"lib/venomq/transport/frame.ex","line_count":58},{"filepath":"lib/venomq/transport/method.ex","line_count":127},{"filepath":"mix.exs","line_count":24},{"filepath":"test/test_helper.exs","line_count":1},{"filepath":"test/transport/data_test.exs","line_count":33}]