Add insert method to Quantity
Add `to` method to Column
Add `to` method tests
Split ``to`` method into a ``quantity`` property and a ``to`` method

They are distinguished by ``to`` requiring a unit where previously it
could be just None
Add changelog entry
Add some documentation on Quantity <-> Table interactions
typo fix pointed out by @astrofrog
Fix for possible rounding problems in test mentioned by @mhvk
removed example that might lead users astray

Because table quantities won't behave right for some things, like
trig functions where the quantity conversion doesn't happen.
Change insert implementation to use np.insert per @mhvk
Add tests of Quantity.insert
Update CHANGES.rst
add equivalencies and fix up docs for to/quantity
Update docs to make it clearer that columns don't always work like quantities
Fix some minor issues in review and travis tests
Added setter for quantity attribte
Fixed some typos pointed out by @mhvk

[skip ci]
Simplify tests by using float quantities
Remove the setter, replace with copy=False and related tests
Fix for missing equivalencies