Added builds folder.

Because the Downloads section has been removed, added a builds folder
for users to download pre-compiled firmware without needing to compile
it themselves.
Added download links to README.
Changed README to markdown
Update README.md
Slow trailing steps fix. Added more defaults.

- Fixed an issue (hopefully) with slow trailing steps after a
triangular velocity profile move. Sets the trapezoid tick cycle counter
to the correct value for an accurate reproduction of the deceleration
curve. Keeps it from arriving too early to the target position, which
causes the slow trailing steps.

- Added Zen Toolworks 7x7 to default settings.

- Updated readme with new edge build.
Readme link to edge build 2012-12-21
Fixed bug with homing and polling at the same time. Updated readme.
Merge pull request #1 from grbl/master

sync to tip