Format the framework restriction in the LockFile
Implemented credential store

As discussed in #312 the credential store will be called when the
authentification is configured, but either username and password
references not existing environment variables or they are both empty
strings. The credential store is an XML file with source, username and
encrypted password. The password is encrypted with the .NET encryption API
on a user account API.
Merge branch 'master' into credentialStore
Fixed merge fails
removed clear text reading password
Use latest fake
Don't fail with NullReferenceException when analyzing nuget.config - references #319
Bump version to 0.11.11
Add failing test for #319
Don't fail with NullReferenceException when analyzing nuget.config - references #319
Bump version to 0.11.12
Add a second nuget.config test
Small cleanup - closes #319
Merge branch 'credentialStore' of github.com:yannisgu/Paket
Fix merge error
Small cleanup - relates to #318
Small cleanup - relates to #318