Remove use of setuptools Feature. Fixes #23
Silently reject old arguments
Bumped version
Fixed missing escaping for keyword arguments.
This is 0.20
Added docs and more tests for new string formatting
This is 0.21
Fixed a formatting error in the readme
idempotent unescape

If we examine the XML spec for entities, we find that ampersand and
space are not allowed characters in an entity name. I've also modified
the unescape function to not modify unexpected inputs (such as &foo;).
This is a common best practice when dealing with layered systems.

http://www.w3.org/TR/REC-xml/#sec-references

EntityRef   ::=   '&' Name ';'
Name   ::=   NameStartChar (NameChar)*
NameChar   ::=   NameStartChar | "-" | "." | [0-9] | #xB7 | [#x0300-#x036F] | [#x203F-#x2040]
NameStartChar   ::=   ":" | [A-Z] | "_" | [a-z] | [#xC0-#xD6] | [#xD8-#xF6] | [#xF8-#x2FF] | [#x370-#x37D] | [#x37F-#x1FFF] | [#x200C-#x200D] | [#x2070-#x218F] | [#x2C00-#x2FEF] | [#x3001-#xD7FF] | [#xF900-#xFDCF] | [#xFDF0-#xFFFD] | [#x10000-#xEFFFF]