Refactoring

[tiabc]

1. Rename methods of the common.JailCell interface:
   CellVM() -> VM()
   CellLoop() -> Loop()
2. Get rid of duplicated code in common.Jail struct methods: Call, JailCell, JailCellVM, GetCell.
3. Get rid of the Jail prefix everywhere in the jail package.
4. Rewrite tests to use jail.NewJailCell instead of jailInstance.Parse(testChatID, ``).
5. In JailTestSuite initialise the internal jail with BaseJS in SetupSuite and initialise a test jail cell in SetupTest. The idea is to make those tests as meaningful as possible avoiding as much extra scaffolding as we can.
6. Refactor all tests to use testify asserts. Tests become twice as short and twice as meaningful.
7. Figure out if one of Jail.GetJailCell and Jail.GetCell could be removed. This looks like rather an architecture issue.
8. See if it's possible to invoke jail.BaseJS in setup methods in JailTestSuite instead of doing it in every test.

As this is a refactoring issue, it must be finished and merged within a day. Please, plan accordingly.
If you have new refactoring notes during implementing these, write them into #200.