Add support for Travis builds

Travis will now build and test the package as Haskell. The current
version of cabal on Travis has a different interface to the latest
testing inferface, so the script installs the latest version in a
sandbox and then uses that to configure and test.

The build status has been added to the readme.
Improve documentation of matching for Accept

The documentation for the 'matches' and 'isMoreSpecific' methods in the
Accept type class was vague and did not completely specify the required
properties of the relations. Now the properties of the relations are
more clearly specified and the primary use case is given as an example.
Add Accept-Language header data and parsing

As language negotiation also involves a degree of matching more
complicated than just equality checking, it is now represented by a
Language datatype with an Accept instance that handles matching and
specificity. Parsing and matching of quality annotated languages is
handled as before, due to the generic type signature of the related
functions.
Add tests for the Language type

The new tests mostly mimic the tests for MediaType. This change includes
a small tidy up of aspects of the existing test suite, as well as
correcting some erroneous behaviour exposed by the tests.
Export Language type and functions from Media

The operations for handle Accept-Language and Content-Language headers
are now exported by the main Media module, along with more specialised
forms of mapAccept and mapContent.
Update documentation and bump version number

The introduction of the Language implementation bumps the pre-release
major version. The documentation has been updated to clarify that the
library is capable of handling more than just mediatypes. The examples
in the readme have been named to help the syntax highlighter.
Allow a Language to be converted to its parts

A Language value could be converted back into its header representation
in a ByteString, but this was not as useful for processing as a straight
list of the language parts. Now the Language module defines a toList
function for Languages.