Started deriving rotating frames.

The plan is to put derivations and learning in the reference book,
leaving the quadcopter as a slightly shorter read with referencing where
appropriate.
Further derivation of rotating frames
Almost completed rotating frames derivation
Continued deriving Euler's equations
Copied Euler angles derivation to reference book
Restructure and contents added to reference book
Completed algebra for rotating equation of motion

Algebra needs checking, but should be just about right.
Merge branch 'master' of https://github.com/pestrickland/notebooks

Conflicts:
	quadcopter_dynamics.ipynb
Revert "Started work on simulation"

This reverts commit 30228a5395f0634202184d7aae339aa1530a2713.
Messy conflicts that couldn't be resolved.

Conflicts:
	quadcopter_dynamics.ipynb
Revert "Started work on simulation"

This reverts commit 30228a5395f0634202184d7aae339aa1530a2713.