Merge pull request #53 from mpickering/master

Fixes to bugs highlighted by round trip testing
Update changelog
TeX reader: use a more general state for the reader monad.
TeX writer:  use \operatorname* for convertible operators.

This fixes one more round-trip test failure.
Regen tests.
Added EPhantom element to Exp.

It's awkward to refer to latex unary operator names in the mathml reader.
TeX writer:  Removed need for fixTree.

Handle accents directly in Under/Over clauses.
(reorderDiacriticals also needed to go because we're removing
EUnary.)

AS a side effect, we have somewhat better output for
over/under accents.
Removed EUnary, added ESqrt and ERoot.

This is cleaner; we don't need to pass around TeX command
names in the AST.
MathML Reader: Improved handling of compound operators