Fix OBJECTS Makefile file extensions.

Previously the OBJECTS files had a .c extension, which meant they would
not be individually compiled. The only reason it worked is because gcc
can take an arbitrary list of source files and compile them "in place,"
(usually in a temporary directory), then link it all automatically.

This commit gives the correct .o extension.
Don't pass sp_session * to spua_forget_blob().

spua_forget_blob() will not require the sp_session to forget itself
(when implemented).
Rewrite how the public interface works.

This commit makes major non-compatible changes to the public interface
of libauth. Specifically, the following changes have been introduced:
1. spua_mode has now been broken up into a struct, spua_config. The
spua_config struct is responsible for storing data necessary for
performing login with each of the authentication methods.
2. The spua_getenv() function has been added to fill in a spua_config
struct with data from the environment. Additionally, in favor of this
approach, the spua_environment flag has been removed. Note that this
uses getenv(), making it non-reentrant and non-thread safe.
3. The spua_libspotify authentication method has been split into
spua_relogin and spua_credentials. Previously, it was planned that
normal login with passwords would be apart of spau_libspotify. This
change makes using sp_session_relogin() and sp_session_login() (without
using a blob) two separate authentication methods.
Make spua_getenv() return the spua_config *.

This commit makes the spua_getenv() function return the spua_config *
that was passed to it (to be filled with environment variables), instead
of returning void.

This is for convenience in the event that it will save a statement.
Implement spua_credentials authentication.

spua_credentials simply logs in with a given username and password.

Additionally, this commit makes the relogin remember and forget
functions apart of spua_credentials. For remembering, this is required
as a sp_session_relogin() can only work after setting remember_me to 1
in sp_session_login() (which is in spua_credentials). So, it makes sense
to make forgetting apart of spua_credentials instead of spua_relogin.