Use of new scopes, general fixes about commands

As the last major step of the scope integration, the command invocation methods were
updated in order to use the new scope architecture. Main changes:

* Pash distinguishes correctly between scripts, functions, and script blocks.
* The ScriptProcessor is now a more general ScriptBlockProcessor that is able to
  process all script-block-like structures (functions, scripts, and script-blocks)
* Depending on invocation method and type, Pash creates new scopes if needed
  and updates the runspace's execution context if a command is (temporarily)
  executed in a new scope. Therefore the CommandManager depends on the
  Runspace, as it needs access to the currently active scope.
* The dot-sourcing implementation was fixed to work correctly. It only supported
  scripts so far. Now dot-sourcing and ampersand invocation work with scripts,
  script blocks, expression resulting in a script name and script blocks stored
  in variables.
* There are new "end-to-end" unit tests that assure scopes work as intended for
  variables and functions
* The way commands are parsed, found and executed changed in order to be easier,
  less redundant and more straight-forward to understand.
  A "command" is now a real command, i.e. one specific word. So Pash strongly
  distiguishes between input, which needs to be parsed, and commands, that
  are searched and processed. Parsable input is a Command with the IsScript flag,
  which is also the desired behavior of the specification, while real commands
  set this flag to false.
  As a result Pash doesn't just try to parse the CommandText, if it didn't find a
  command, but knows exactly when to parse input and when to search for a command.

With these changes, Pash fully supports scopes.
Merge pull request #182 from sburnicki/scopes_step3

Third step of scope integration: Use of scopes, general fixes/cleanup about commands