0.27.0-alpha013

Allow to reference .props and .targets files - #516
Create a install function in the api which takes a paket.dependencies file as text - #576
Don't internalize in paket.exe during ILMerge
Allow to download from pre-authenticated MyGet feed - #466
BUGFIX: Fix paket install --hard for FSharp.Core - #579
BUGFIX: paket convert-from-nuget ignores casing when looking for nuget.targets - #580
BUGFIX: paket install correctly parses HTTP references - #571
BUGFIX: paket.dependencies parser fails if tokens are not valid
COSMETICS: Prerelease strings are checked if they contain operators