Improved handling for TeX log files

[Sharpie]

It would be nice to store the log files of failed runs in a list, say the last 5 bad ones. Then instead of dumping pages of text to the console, we would just print an error message and say something like "run tikz_showlog to see the logfile".

This could also be extended to keeping every log if the device is built in debug mode which would help with package development.

[Sharpie]

Definitely need to get to this---upgraded to MacTeX basic 2011 and didn't have preview or collection-fontsreccomended installed. The resulting TeX logs in error messages produced by tikzDevice were absolutely worthless.

[cameronbracken]

This could even be extended to a general tex log parser...

[Sharpie]

One thing I have thought about is loading the silence package by default to cut down on the log verbosity.