Merge commit 'b21e8b2af3594286d6a8ece7bdd11bda6cedfbfb'

Conflicts:
	libsmo/include/user_course.hpp
Added a bit new API that I found will be needed, some work on making smo-gtk use the SQL backend
tmp
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More work on making the frontend work with the SQL backend, removed grades table
Compiling with sql backend now works, next TODO linking
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Fixed linking, next TODO make it work...
Changed the return type of smo_user::get_learn_periods() and implemented it
Implemented some functions, little API change on the create method
Another API change and some work on the consistency in libsmo
Tiny API change, a bit implementing SQL stuff
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Some implementation + little API corrections
Some implementation of learn_period methods
Merge branch 'master' of https://github.com/jonessen96/libsmo

Conflicts:
	src/learn_period.cpp
Updated EER diagram
Implemented all the missing learn_period methods
Merge commit '843a644b617a2262a7677ef98d3a06f55b3519a4'

Conflicts:
	libsmo/include/learn_period.hpp
	libsmo/include/lesson.hpp
	libsmo/include/smo_user.hpp
	libsmo/src/grade.cpp
	libsmo/src/learn_period.cpp
	libsmo/src/lesson.cpp
	libsmo/src/smo_user.cpp