Crash when emailing or exporting PNG of large graph

[glen.low@…]

When you try to email or export PNG of a large graph, Instaviz crashes with a low memory error. This is repeatable with e.g. awilliams.gv from the Graphviz samples, which produces a 30M bitmap.

[glenlow]

(In [173]) Fixed crash with low memory when emailing or exporting PNG of large graph (now uses temp scratch file to back memory allocation if bitmap context would be too large, fix #106). Fixed long session start appearing to hang without progress bar coming up. Fixed crash with low memory when progress bar needs to be displayed (check whether view is still allocated when attaching action sheet).