Added norm function that respects units
WIP: Moved twobody.py to package
This helps splitting functions and organizing the plotting module,
already imported but yet to be implemented.
Created plotting module
Fixed setup.py after 0a20765
Now the extension modules are placed in the correct directory.
Updated example notebooks
Expanded user guide, small fixes
Added (untested) apply_maneuver method
BUG: Fixed direction of impulses in maneuvers
Both in hohmann and bielliptic the magnitude of the impulses was
correct but not their direction. No tests were detecting this
failure, which was discovered checking the eccentricity of a final
orbit.
Added some more tests
Added Plyades as source of inspiration
Added indexing capability for Maneuver
Added new properties to State
Revamped OrbitPlotter to allow several plots
Now the plane of the first plotted state is set as the plotting
plane for the rest of the states. Leaving 3D plots apart, this
was the most sensible solution I could come up with to have
nice and informative 2D orbit plots.
BUG: Fixed circular import between plotting and twobody
If I want to provide a method on `State` to plot, I think I have
to slightly break the consistency of the code in some way.
Either I totally separate plotting and twobody, hence requiring
the user an extra import, or I do this.
Finally removed plot methods from State
This way I avoid all circular imports at the cost of losing one
handy method. Now this:

>>> from poliastro.examples import molniya
>>> molniya.plot()

is replaced by this:

>>> from poliastro.examples import molniya
>>> from poliastro.plotting import plot
>>> plot(molniya)
BUG: Fixed several edge cases for orbits
Basically rv2coe is ill-conditioned for equatorial and/or circular
orbits, which have low or zero values of inclination and eccentricity.
This has now been fixed.
Allow apply_maneuver to return intermediate states
Also fixed epoch of these, which was not being assigned correctly.
Finished user guide
Added more tests, pep8 fixes