Provide macro @libcall to simplify ccall's

[emmt]

A macro is provided to automatically use the correct library based on the prefix of the function. This makes the code a bit more readable and less prone to errors. The macro Cairo.@libcall can be used in other packages to simplify calling a Cairo/Pango function.

[coveralls]

Coverage decreased (-1.06%) to 91.574% when pulling 823c57e on emmt:eric-patch into 781dae0 on JuliaGraphics:master.

[lobingera]

"and less prone to errors" - which errors?

[emmt]

Well when the code is more clear and easier to read, I think that it less likely to make it wrong. I did not meant that that there were errors ;-)

[asinghvi17]

This now exists in Base's @ccall macro.

[ViralBShah]

Agree this should be done with @ccall