Add reversible romanization method

[blagarde]

Neither of the current romanization methods are fully reversible.

Effort seems to have been put towards making to_kunrei reversible, even though this forces it to not strictly follow the Kunrei scheme:
to_kunrei:

ち -> ti
てぃ  -> texi (should also be 'ti')

(See "ティーム" vs. "チーム", http://en.wikipedia.org/wiki/Kunrei-shiki_romanization)

A reversible method is a useful thing to have, however:

1. The current function probably should be named differently because it doesn't follow the Kunrei scheme exactly (how about "reversible", or "romkan"?)
2. The current to_kunrei function is nearly there, but still not perfectly reversible:

to_kunrei:

ぢ -> dyi
でぃ  -> dyi

The Hepburn function is also not reversible, but this is a known property of the scheme.

to_hepburn:

ず -> zu
づ -> zu