Added sublevel support.

Closes #52.
Do not patch level-writestream after it has been subleveled.
Fixed sublevel tests in the browsers.
Merge pull request #53 from mcollina/sublevel-support

Added sublevel support.
Bumped v0.7.2. Update HISTORY.md.
minified browserify build for v0.7.2.
Removed useless file lib/streamwrapper.js.
Faster SortJoinStream.
Setting highWaterMark to 16 on all read streams.
added javascript markdown highlighting
Merge pull request #56 from boxxxie/patch-1

added javascript markdown highlighting
From 36.000 triples per second to 50.000 triples per second.
Removed solutionsfilterstream.js as it is useless.
Fixed generation of false values in the keys.

Closes #55.
Updated README to explain how to update triples.

Closes #54.