Add floating point numbers

Summary:
This revision adds lexing and parsing of floating point numbers to
tinyquery.  I debated whether it would be better to add them as part of
the NUMBER token type or add a different FLOAT token type.

I chose the separate token type after a brief survey of various sources
of lex documentation/examples, which didn't explicitly recommend one or
the other, but did show samples where floating-point numbers and
integers were separate token types.  This strategy also seemed cleaner
to me than having a single token type match either and then figuring out
whether it was an int or a float afterwards.

Fix existing lint error.

Test Plan:
- add tests to the lexer and parser for floating point numbers
- `nosetests`

Reviewers: alan

Reviewed By: alan

Subscribers: jace

Differential Revision: http://phabricator.khanacademy.org/D11079