Implementation of New-Item for FileSystemProvider
Basic Out-File
Fixes NPE in some cases
Small fixes to match PS behavior
Merged subexpressions of expression_with_unary_operator in one to speed up grammar parsing
Finished joining most unary operator expressions

With the new implementation Pash starts significantly faster than before
(only ~39% of the time for starting and exiting).

All expressions with unary operators have been joined in one expression to achieve this.
The cast expression, however, had to stay there as a separate rule. Although I thought
that joining it wouldn't make a semantical difference for the grammar, a double-cast
like [int][string] wouldn't be parsed anymore.
Fixed path for the reference tests in Pash.proj.

Building with msbuild or xbuild now works again.
Merge pull request #235 from ygra/ref-tests-project

Fixed path for the reference tests in Pash.proj.
Merge pull request #234 from sburnicki/startuptime

Significantly improved startup time
Merge pull request #233 from sburnicki/filesys_provider

FilesystemProvider New-Item support, basic Out-File support
Update README and CONTRIBUTING
Merge pull request #237 from Pash-Project/update-readme-and-contributing

Update README and CONTRIBUTING
Correct conversion to number, unified arithmetic operators

- Conversion of operands to numeric values as in the PS specification.
- Common method for arithmetic operations
- ReferenceTests for Additions
Tests for subtraction and multiplication, implementations
Support for multiplication, division and remainder
Support for expressions like 1/3=0.333