Merge pull request #43 from rsrock/master

Fixes initial canvas display issues on the Mac
merge
simplify tk_wrapper so we can build against MacPorts and system Tk, without special headers
rewrite tk_wrapper in pure-c (no objective-c)
remove Canvas.front by refactoring some code. this lets us cleanup after drawing to the screen rather than leaving around a bunch of stale state
eliminate tk_wrapper
fix tcl_result call at startup
some windows fixes for previous commit
cleanup more mac drawing state between calls to render_to_cairo
render to a Cairo.surface_create_similar for better performance
fix flip geometry for child elements
Merge pull request #46 from JuliaLang/jn/cleanup

cleanup
Add /usr/local/opt to library search dirs