Fixed bug with Bool arrays.

While the RosBinary instance handles serialization of Haskell Bool
values to and from a single byte, the Storable instance for Bool uses
four bytes. A problem then occurs when we want to use a Storable Vector
to hold boolean values. If we receive four bytes for an array of boolean
values from a peer, that is four distinct booleans, but the Storable
instance for Bool wants the Vector of length four to use 16 bytes.

Instead, we simply map the Ros bool type to a Haskell Word8 for now.

Fixes #15
Bumped haxr dependency to address roscpp compat.

Some communication with roscpp nodes was causing trouble for roshask due
to an implicit string type annotation in XML-RPC. According to the spec,
an XML-RPC value such as "<value></value>" should be interpreted as an
empty string. haxr was instead looking for
"<value><string></string></value>" for such values.

I patched haxr to accept unwrapped text as a string value.

Fixes #14