Track XML C lexer state in C only.

Instead of storing "act" and "cs" as an instance variable they (along with some
other variables) are now stored in a struct. This struct is attached to a lexer
instance using the (crappy) Data_Get_Struct/Data_Wrap_Struct API.
Track Ragel call stacks in the Java lexer.

This will be needed for the upcoming string lexing changes.
Better support for lexing multi-line strings.

When lexing multi-line strings everything used to work fine as long as the input
were to be read as a whole. However, when using an IO instance all hell would
break loose. Due to the lexer reading IO instances on a per line basis,
sometimes Ragel would end up setting "ts" to NULL. For example, the following
input would break the lexer:

    <foo class="\nbar" />

Due to the input being read per line, the following data would be sent to the
lexer:

    <foo class="\n
    bar" />

This would result in different (or NULL) pointers being used for building a
string, in turn resulting in memory allocation errors.

To work around this the string lexing setup has been broken into separate
machines for single and double quoted strings. The tokens used have also been
changed so that instead of just "T_STRING" there are now the following tokens:

* T_STRING_SQUOTE
* T_STRING_DQUOTE
* T_STRING_BODY

A string can have multiple T_STRING_BODY tokens (= multi-line strings, only the
case for IO inputs). These strings are stitched back together by the parser.

This fixes #58.