…ing savings

This changes the default data for mime-types from a JSON
file to a set of text files, with one line per mime type, and one
file per mime type attribute.  This only loads the content_type and
extensions for mime-types by default, reducing default memory usage
of mime-types from over 22MB to just over 2MB.  Additionally, it
speeds up requiring from 0.38 seconds to about 0.10 seconds.

Even when loading all of the attributes for the mime types, the
memory usage of mime-types stays under 11MB, though load time
increases to 0.52 seconds.  I think that is an acceptable trade off.

If the txt files are shipped instead of the json file, there is also
a disk space savings of 167KB (392KB instead of 560KB).

rake convert:yaml:txt has been added for converting the yaml files
to the necessary txt files.  In order to get the conversion to work,
I had to make convert.rb turn on lazy loading for mime types.

This should be completely backwards compatible if a path is given
containing the types.  The only current backwards compatibility
difference is for the "text/directory - DEPRECATED by RFC6350" mime
type, which looks to me like an invalid mime type anyway, and
probably should be removed.

Use a single string for each encoding.  This decreases the number of
encoding strings stored by default from 1909 to 4, saving about 700KB
of memory by default on 64-bit MRI.

While here, reduce object allocations while loading data from the
mime type txt files.

This was causing the test-order dependent bug on JRuby.
The clear_cache_file code was taken from the cache test.