Allow variables inside double quoted strings.

Example: "$foo"
Implement AST for variables inside an expandable string.

Simple variables inside a double quoted string are added to the
expandable string's nested expressions.
Implement script extent for parse tree nodes.
Add script extent for nested expressions inside expandable strings.
Expand variables inside double quoted strings.

"a = $a"
Update authors.
Merge branch 'master' into expandable-strings

Conflicts:
	AUTHORS.md
Merge pull request #177 from mrward/expandable-strings

Expand variables inside double quoted strings
Scopes: Functions, Aliases, Refacoring

This is the second big step of implementing scopes in Pash.
The major changes are the following:
 * FunctionIntrinsics and AliasIntrinsics were introduced, s.t. function and aliases
   are handled analogously to variables and drives.
   Note: All the *Intrinsics classes need to be a little extended if the Providers /
   cmdlets are updated to use them. But this shouldn't be hard any more, because
   of the next point.

 * The class SessionStateScope, which was introduced in the last step, was changed to
   be generic and always refer to a specific type of items.
   This approach was chosen, because the *Intrinsics classes are all specified a little
   differently with different details. To avoid a lot of duplicate code, the core
   methods are now directly in SessionStateScope, only the details (or wrapper functions)
   are exposed in the *Intrinsic classes.

 * The interface IScopedItem was introduced in oder to make AliasInfo, PSVariable, PSDriveInfo,
   and FunctionInfo all usable by the new SessionStateScope implementation.

 * The tests however still refer to the *Intrinsics classes, though they share mostly common
   functionality. However, these classes count in the end and bring slightly differences,
   so this makes sense.

 * Some Exception classes were implemented in order to be used.

 * First necessary changes in the cmdlets were done, in order to be able to integrate the
   new architecture. However, they still need some refinement in order to support all features

 * The CommandManager now always need an ExecutionContext in order to instantiate. This is
   necessary in the new architecture and does make sense, as the SessionState is part of
   the ExecutionContext and it provides information about available aliases and providers.
   Therfore the provider intialization was moved to SessionStateGlobal, for now.
   Later on, also the Providers should be handled by SessionState.Provider (ProviderIntrinsics,
   see specification),  but are no scoped items.

All tests run without errors, on both windows and linux. I also tested some basic work in Pash,
and didn't face any problems.

Upcoming steps:
 * Make sure new scopes are created when scripts, functions or scriptblocks are executed
 * Take another look at the existing cmdlets and refine some
 * Make functions work with parameters
Merge pull request #178 from sburnicki/scopes_step2

Second step of scope integration: Functions, Aliases, Refacoring