Fixes #215 - Real literal tests are broken for non-US locales

Run real and decimal literal tests with en-US locale.
Merge pull request #216 from ygra/reference-tests

Some build system improvements to reference tests
Fix parsing of real literals on non-US locale.
Fix incorrect conversion of double in string on non-US locale.

Add tests for real literal output on non-US locales.
Merge pull request #217 from mrward/real-literal-different-locales

Real literals on different locales
Change default script scope to global.

With Microsoft's PowerShell the following:

    Pipeline.Commands.AddScript(script);

is equivalent to:

    Pipeline.Commands.AddScript(script, useLocalScope: false);

Added a reference test for check this behaviour.
Fix broken assert

First, it was asserting the wrong expression, which had no value. Fixed.

Second, Debug.Assert is ignored when NUnit runs in Travis-CI. Added a TraceListener to make it that fail.
Merge pull request #220 from mrward/script-default-scope

Change default script scope to global.
Support reading/writing environment variables using $env

Implemented support for:

    $env:userprofile
    $env:MyVariable = "MyValue"
Support getting child items of Env: drive.

The following commands now return all environment variables:

    dir env:
    dir env:\
    Get-ChildItem env:
    Get-ChildItem env:\
Support individual environment variables from Env: drive.

The following commands now return an environment variable:

    dir env:userprofile
    dir env:\userprofile
    Get-ChildItem env:userprofile
    Get-ChildItem env:\userprofile
Environment variables can now be selected after changing to Env drive.

Support the following:

    cd env:
    dir userprofile
Merge pull request #222 from mrward/environment-provider

Fixed some parts of the Environment provider
Add cmdlet Split-Path and tests
Path cmdLet tests no Platform
Merge pull request #223 from darth-walsh/splitpath

Split-Path cmdlet
Renamed PS reference test project to reflect brand correctly
Refactoring: Moved SetUp and TearDown methods to RefererenceTestBase
Moved ReferenceTestInfo to the correct directory.
Merge pull request #199 from sburnicki/rename_reftests

Renamed PS reference test project to reflect brand correctly