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
Merge branch 'credentialStore' of github.com:yannisgu/Paket
Fix merge error
Small cleanup - relates to #318
Small cleanup - relates to #318
Small cleanup - relates to #318
Fixed travis build from #318

After merging #318, the tests were failing on travis due a mono bug, which
doesn't set the correct permissions on the keystore. When manually
creating the keystore and setting the correct permission the build will
pass.
Add Fuchu test - references #323
Merge pull request #324 from yannisgu/fixCredentialStore

Fixed travis build from #318
Trace warning when we replace NuGet.exe with NuGet.CommandLine - closes #320
Bump version to 0.11.13
Fix security issue in credential store

As pointed out by @hef the initial implementation of the credential store
had a security issue to to constant entopy bytes. Now the entopy bytes are
generated for each source and saved next to the source.
removed not needed type
Merge pull request #325 from yannisgu/securityFixCredentialStore

Fix security issue in credential store
Prepare for #327
Merge branch 'master' of https://github.com/fsprojects/Paket
BUGFIX: Paket timing was incorrect -closes #326