Read from user defined Rakefile

Currently, Rakefile that created by motion create is fixed.
we always add TestFlight setting, some require statement, etc,,, to generated Rakefile each motion create.
it's a pain.

I thought that motion create use my defined Rakefile!

on this commit, Read Rakefile placed in ~/.rubymotion/

in Rakefile, #{$motion_libdir} and #{app_name} are occurred then replace to their variable's values.