Tests/enhancement to close #687 and #696

A list of changes includes:
* [x] ``GetCustom``, ``PostCustom``, ``DeleteCustom``, ``PostFormCustom`` methods returning request wrapper which can be ``Send()``ed have been added.

  *Note: ``..Custom`` methods family gets ``uri`` as first input parameter as opposed to ``Get``, ``Post``, etc. which expect ``path`` instead.*
* [x] ``MakeMultipartRequest`` replaced by ``PostFile`` and ``PostFileCustom`` which expect fieldnames and their values, and filenames and their paths as input, open the files and send/return a multipart request.
* [x] ``upload`` sample has been added for demonstration of avatar upload and its validation (file size / format / resolution)
* [x] ``upload`` sample covered by some tests.
* [x] ``persona`` sample's tests were modified to reflect the changes in test framework.
* [x] Number of http requests to third party services by ``persona`` sample has been decreased to speed up the tests execution.
* [x] ``AppTest`` is replaced by ``*AppTest`` in skeleton of ``tests/testapp.go`` to let testrunner show Response info (#725).