Added cmd option to add credentials

With `paket config add-credentials http://dummy` credentials will be added
to the user-config file.
Add WindowsAzure.Storage.nuspec - references #462
Small cleanup
Use Max strategy for paket outdated --ingore-constraints - references #446
Parse >= FrameworkRestrictions
Can parse multiple FrameworkRestrictions
ApplyAtLeast restriction
ApplyAtLeast restriction
Group FrameworkRestrictions
Invalidate the cache
More grouping of the FrameworkRestrictions
Clear all warnings
Grouping to AtLeast
Implement Between
Fix Math.Numerics
Merge pull request #465 from fsprojects/groups

WIP: Try to solve #461 with FrameworkRestrictions
RELEASE_NOTES
Bump version to 0.20.0-alpha001
Dogfood
cleanup RELEASE_NOTES