add tagging functionality to pester describes

cli usage:
pester <specific test> -Tags "<tag1><space><optionaltag2>"

describe usage:
Describe "Add-Numbers" -Tags 'adder','calculator' {...}
Use PowerShell syntax highlighting in the readme file.
Merge pull request #36 from ming13/readme-syntax-highlighting

Use PowerShell syntax highlighting in the readme file
added support for should.not_be when writing tests. In order to test this, an additional test was added to the calculator to make sure that the sum was not a specific number.
Merge pull request #38 from stack72/master

Support for should.not_be
adding some pipeline assertion functions
adding a changelog to better document releases
succumbing to my OCD. fixing whitespace, and making long lines into multi-line statements
moving xsd out of the root
renaming default.ps1 to a more clarifying name build.psake.ps1
moved png to pester.github.com repo
Merge branch 'add_tagging' of git://github.com/rismoney/Pester into rismoney-add_tagging

Conflicts:
	Functions/Describe.ps1
adding tags to the example spec
adding CHANGELOG item for describe tagging
working on issue #9. unfortunately this will break the build
still working on #9. Test simply spits out the extra keys that exist when a test is running
moving testResults onto $pester scope. Still working on #9
looks like some variables will always leak, but essentially this fixes #9. will leave scoping of functions for now.
updating CHANGELOG for scoping fixes
trying out a new assertion syntax