Support windows credential store/gnome-keyring to store passwords

[Megous]

Megatools can save mega.nz account password in windows credential store/gnome keyring, instead of in .megarc.

Password can be saved after using interactive prompt.

Example:

$ megals -u [email protected] --persist-password
Enter passwword: *********

$ megals -u [email protected]     # uses saved password
/Root
/....

Credential store API: http://stackoverflow.com/questions/9221245/how-do-i-store-and-retrieve-credentials-from-the-windows-vault-credential-manage