Added mailimap_has_xoauth2() to detect whether XOAUTH2 is available.

Improving OAuth2 documentation a little bit

Adding some additional info on how to retrieve a user's e-mail address
using Google's UserInfo API

Adding OAuth2 authentication support

Implements two functions:
mailimap_oauth2_authenticate: Use to to send the authentication token
to the server and actually authenticate with it.
mailimap_oauth2_authenticate_send Builds and sends the authentication
string in the form of base64("user=" {User} "^Aauth=Bearer " {Access
Token} "^A^A")`
This has been tested against Gmail.

Fixed MSVC build

Added some conditionals, fixed vcprojects to include added source files,

updated build_headers.bat to hopefully make it less fragile,
changed a few declarations from middle of function to beginning (c
standard)

NULL declaration is missing on Ubuntu